My 24 years' old goalkeeper is going to finish his 7th season with 266 appearances and I've never tanked.
I will try to put the mark on 400. He's one of the fastest trainers I had and the highest rated GK I've seen, however I don't have the means to keep him 5*/6*/7* forever as he'll become slower. With 6 stars his skill progress is still around 95% per game and he gets over 50% with hard training. If he reaches 6 stars again next season, I'll probably be able to keep him for 4 seasons more, maybe 5. It will all depend on his training speed in the future.
I bought this guy from a manager playing 2 levels above me. I don't know if he was from the Youth Academy or if the previous owner bought him in auction and resold, as he was near 6 stars when I got him at 18.