The expansion clubs were built on expansion drafts in which the other clubs had to make players available. The rules did not remain constant for the various drafts of 1960, 1961, 1968, 1976, 1992, 1997. There was also a small supplementary draft for Houston and the Mets in October 1963.
Anyway, the average first year record of the 14 expansion clubs during this time frame was 60.6-100.9 (some teams did not play the full 162 game schedule or there were ties). The range was from 40 wins (Mets - 1962) to 70 (Angels - 1960) and it took an average of 6.5 seasons for the clubs to reach at least the .500 mark in any one season (the Angels and the Diamondbacks each achieved a winning record in their second season while it took Seattle 14 seasons. In all, it took nine of the clubs at least six seasons to get to at least a .500 mark but not many of them kept that up).