I would say 3-5 years on a good sized peice.
I have very little expereince hunting around crops, I end up in some big woods quite alot, with no real farming around.
Other things I watch for are blowdowns and beaver dams.
I like both of these also.
Our lease which we had two seasons (don't even want to go into it) was the first time I was able to hunt around farm land. It was 220 acres of logging company land the connected into over 15,000 acres of State lands that weren't hunted that hard. On two sides there were fields with crops.The first year there were so many acorns with water around that the deer didn't have to move 100 yards in a day if they didn't want to. We also aquired it about 5 days before bow season.the first season my friend took a 138 inch 9 pt and I took an 8 that had been shot in the head that went about 110'. Some idiot had shot him through the snout.
The second year I did a ton of Turkey hunting on it and started to get a hold onarea I had a good feeling about,it broke down a steep ridge with a corn field at the top pond in the bottom with some knarly thick bedding near by,and alot of old rubs. Well to make a long story short evrything came togther and I blew it on a really nice ten point.
Given a couple mopre seasons I think I would have been able to really get a feeling for the what and when on that piece.
This season I got permission on 175 acres that has six fields but no crops, and once again we had a boomeracorn crop.