In 1925, three land-based Hawker fighter aircraft were purchased to be based in Ringsted. A modified version of the Hawker Woodcock, the Hawker DANKOK, was initially delivered from the UK while a following series of 12 aircraft were license-built by the Orlogsværft. In 1928 the naval air service procures six Heinkel HE 8 floatplanes, with another 16 to be license-built by the Orlogsværft. As Germany were not allowed to produce military aircraft, the planes were labelled as mail planes, but they could easily be refitted with dual machine guns, radio equipment and a capacity for eight bombs. The Heinkel had an unexpected arctic employment in the early 1930s when a dispute over East Greenland caused the first deployment of aircraft at Greenland along with three naval ships. Following the settlement of the dispute between Denmark and Norway the Heinkels were used in the efforts to map the frontiers of the island. In 1933, two Hawker Nimrod were acquired to keep pace with the rapidly evolving technology of naval aircraft in the interwar period. A further ten planes were to be produced under licence at the Orlogsværft. The new aircraft made the air base in Ringsted inadequate and the naval air service were moved to the Avnø peninsula at southern Zealand.
Question: How many years passed when six Heinkel HE 8 floatplanes were procured?
Answer: 3