Hank Pym becaqme a scientific Avenger in Avengers West Coast #21. He basically remained with the team right up until #74, when he officially left the WCA, however he came back for Operation Galactic Storm (circa AWC #80-82), still as Dr. Pym the Scientific Adventurer, going as far as giving Hawkeye his growing serum allowing the archer to become Goliath II for the duration, and stayed to lead the West Coast Reserve team in #83. He than appeared during the Ultron Storyline (circa #89-91) as a mere scientist, hardly taking part in any action and when he showed up agian, in #101-102 was already as Giant Man. So, I Guess I'd count from #21 to #91, though he was not around much after #74. Little over five years.
Peace