It really depends on a few things.. how fast a trainer he is, how many matches he gets to play (if you are in CL and Cup they get more SP in matches)

But on my alt Teams I expect 4 or 5 seasons from "core" players. There is high q old guys turnover happening too, but the best ones stay pretty long if you use every bit of possible training on them; these are from a 35T team:

The best, bought when 18 or 19 (so long ago I cant remember.. )
high cost when I got him and worth it. I will keep him until he retires. Click image for larger version.  Name: Vizz.jpg  Views: 15  Size: 82.6 KB  ID: 24315

Wasn't highest $/Q when I got him or he might have more time with my team: Click image for larger version.  Name: E James.jpg  Views: 13  Size: 84.5 KB  ID: 24314