Question: The Battle of Forum Julii was fought between the armies of the rival Roman emperors Otho and an Emperor that served for how many months?

Context articles: List of Roman imperial victory titles
Vitellius
Otho
Battle of Abritus
Battle of Forum Julii
Imperator
Forum Iulii
Concordat of Worms
Battle of Chrysopolis
Battle of Locus Castorum

This document is a list of victory titles assumed by Roman Emperors, not including assumption of the title "Imperator" (originally itself a victory title); note that the Roman Emperors were not the only persons to assume victory titles (Maximinus Thrax acquired his victory title during the reign of a previous Emperor).
 In a sense, the Imperial victory titles give an interesting summary of which wars and which adversaries were considered significant by the senior leadership of the Roman Empire, but in some cases more opportunistic motifs play a role, even to the point of glorifying a victory that was by no means a real triumph (but celebrated as one for internal political prestige).
 For a more complete list of the Emperors themselves, see List of Roman Emperors.
Vitellius (Latin: "Aulus Vitellius Germanicus Augustus" ; 24 September AD 15 – 22 December AD 69) was Roman Emperor for eight months, from 16 April to 22 December AD 69.
 Vitellius was proclaimed emperor following the quick succession of the previous emperors Galba and Otho, in a year of civil war known as the Year of the Four Emperors.
Otho ( ; Latin: "Marcus Salvius Otho Caesar Augustus" ; 28 April 32 – 16 April 69) was Roman emperor for three months, from 15 January to 16 April 69.
 He was the second emperor of the Year of the Four Emperors.
The Battle of Abritus, also known as the Battle of Forum Terebronii, occurred in the Roman province of Moesia Inferior (modern Razgrad, Bulgaria) probably in July, 251, between the Roman Empire and a federation of Scythian tribesmen under the Goth king Cniva.
 The Romans were soundly defeated, and Roman emperors Decius and his son Herennius Etruscus were both killed during battle.
 They became the first Roman emperors killed in a battle by a foreign enemy.
The Battle of Forum Julii was fought between the armies of the rival Roman emperors Otho and Vitellius forces in early AD 69.
 It is described by Tacitus in his "Histories" at 2.14-15.
 The exact location of this battle is not mentioned by the historian, however, he alludes to its taking place in Gallia Narbonensis, possibly near modern Fréjus.
The Latin word imperator was originally a title roughly equivalent to "commander" under the Roman Republic.
 Later it became a part of the titulature of the Roman Emperors as part of their cognomen.
 The English word "emperor" derives from "imperator" via Old French "Empereür".
 The Roman emperors themselves generally based their authority on multiple titles and positions, rather than preferring any single title.
 Nevertheless, "imperator" was used relatively consistently as an element of a Roman ruler's title throughout the principate (derived from "princeps," from which "prince" in English is derived) and the dominate.
Forum Iulii, Latin meaning 'marketplace of Julius', or Forum Julii in modernized spelling, can refer in Latin to the following cities:
The Concordat of Worms (Latin: "Concordatum Wormatiense" ), sometimes called the Pactum Calixtinum by papal historians, was an agreement between Pope Calixtus II and Holy Roman Emperor Henry V on September 23, 1122, near the city of Worms.
 It brought to an end the first phase of the power struggle between the Papacy and the Holy Roman Emperors and has been interpreted as containing within itself the germ of nation-based sovereignty that would one day be confirmed in the Treaty of Westphalia (1648).
 In part this was an unforeseen result of strategic maneuvering between the Church and the European sovereigns over political control within their domains.
 The King was recognised as having the right to invest bishops with secular authority ("by the lance") in the territories they governed, but not with sacred authority ("by ring and staff").
 The result was that bishops owed allegiance in worldly matters both to the pope and to the king, for they were obliged to affirm the right of the sovereign to call upon them for military support, under his oath of fealty.
 Previous Holy Roman Emperors had thought it their right, granted by God, to name Church officials within their territories (such as bishops) and to confirm the Papal election (and, at times of extraordinary urgency, actually name popes).
 In fact, the Emperors had been heavily relying on bishops for their secular administration, as they were not hereditary or quasi-hereditary nobility with family interests.
 A more immediate result of the Investiture struggle identified a proprietary right that adhered to sovereign territory, recognising the right of kings to income from the territory of a vacant diocese and a basis for justifiable taxation.
 These rights lay outside feudalism, which defined authority in a hierarchy of personal relations, with only a loose relation to territory.
 The pope emerged as a figure above and out of the direct control of the Holy Roman Emperor.
The Battle of Chrysopolis was fought on 18 September 324 at Chrysopolis (modern Üsküdar), near Chalcedon (modern Kadıköy), between the two Roman emperors Constantine I and Licinius.
 The battle was the final encounter between the two emperors.
 After his navy's defeat in the Battle of the Hellespont, Licinius withdrew his forces from the city of Byzantium across the Bosphorus to Chalcedon in Bithynia.
 Constantine followed, and won the subsequent battle.
 This left Constantine as the sole emperor, ending the period of the Tetrarchy.
The Battle of Locus Castorum took place during the Year of the Four Emperors between the armies of the rival Roman emperors Otho and Vitellius.
 Locus Castorum was a village that existed in the 1st century Roman Empire roughly 15 kilometers from Cremona.
 It was also referred to as "the Castors" and "at Castor's."
 The village may have been the location of a temple to the Gemini twins, Castor and Pollux.