I always have a full stable of 60 horses.
I have one game which I started when the game had been out a month or so. I am in season 95. I left the game for 2 months when Football Manager 09 was released though and then came back to it.
Currently it takes me a day to do a season but this is because trialing can take upto 2 hours. I am also looking after my lad in between

which is probably why it takes me soo long.
I know trainers who have 2-4 different games that are both over the 60 season mark.
Dont be worrying about how many seasons other trainers are doing. It doesnt take long to produce good horses if your breeding barn is filled with good horses.