With a major-label budget, the Paul Lani-produced So Far, So Good... So What! took over five months to record. The production was plagued with problems, partially due to Mustaine's struggle with drug addiction. Mustaine later said: "The production of So Far, So Good... So What! was horrible, mostly due to substances and the priorities we had or didn't have at the time." Mustaine clashed with Lani, beginning with Lani's insistence that the drums be recorded separately from the cymbals, an unheard-of process for rock drummers. Mustaine and Lani became estranged during the mixing, and Lani was replaced by Michael Wagener, who remixed the album.So Far, So Good... So What! was released in January 1988 and was well received by fans and critics. The album featured a cover version of the Sex Pistols' "Anarchy in the U.K."; Mustaine changed the lyrics, later saying that he had simply heard them incorrectly. To support the album, Megadeth embarked on a world tour, opening for Dio in Europe and then joining Iron Maiden's Seventh Tour of a Seventh Tour in the United States.In June 1988, Megadeth appeared in Penelope Spheeris' documentary The Decline of Western Civilization Part II: The Metal Years. The documentary chronicled the Los Angeles heavy metal scene of the late 1980s, and Spheeris, who had directed Megadeth in the video for "Wake Up Dead", decided to include them to feature a more serious band in contrast to the glam metal groups. Mustaine remembered the film as a disappointment, as it aligned Megadeth with "a bunch of shit bands".In August, the band appeared at the Monsters of Rock festival at Castle Donington in the United Kingdom, performing to an audience of more than 100,000. One show featured a guest appearance by Metallica drummer (and Mustaine's former bandmate) Lars Ulrich. The band was added to the Monsters of Rock European tour, but left after the first show due to Ellefson's drug problems, for which he was treated immediately.Shortly after the Monsters of Rock appearance, Mustaine fired Behler and Young and canceled Megadeth's Australian tour. "On the road, things escalated from a small border skirmish into a full-on raging war," Mustaine later recalled. "I think a lot of us were inconsistent because of [drugs]." During the tour, Mustaine noticed problems developing with Behler and brought in drummer Nick Menza as Behler's drum technician. As with Samuelson, Menza was expected to take over if Behler could not continue the tour. Menza replaced Behler in 1989. Young's dismissal resulted from Mustaine's suspicions that he was having an affair with Mustaine's girlfriend, an allegation Young denied.The band was unable to quickly find a suitable replacement for Young. At this time, Megadeth recorded a cover version of Alice Cooper's "No More Mr. Nice Guy" which appeared on the soundtrack to the Wes Craven horror movie Shocker. The video was directed by Penelope Spheeris, who recalled the filming as a "Herculean task" as Mustaine was unable to play guitar because of his drug addiction. During the March 1989 auditions for a new lead guitarist, Mustaine was arrested for driving under the influence and possession of narcotics after crashing into a parked vehicle occupied by an off-duty police officer. Mustaine entered court-ordered drug rehabilitation shortly afterwards, and became drug-free for the first time in ten years.

Question: What made Mustain replace his drummer with the new one?
Answer: noticed problems developing with Behler

Question: how long had Mustaine been using drugs?
Answer: ten years