Depends on many factors. The wood of the trees would probably still be there but disintegrated for the most part. The branches and leaves only last about 2 weeks and the tree will last for years but it will lay down, move with current, bury under silt, etc.
Now if others fish this spot and put their own piles on top of them (that is usually what a lot of people do) then the pile will be newer and still good. You have to scan with your electronics to be sure.
Scan, scan and scan some more. Some people place their piles close to but not on the same spots.
If the crappie are used to staging on a certain brush pile and you add other brush piles next to it then you will get some stragglers on your pile until they get used to it and will be on it more often as time goes by.
Best bet for me was to place my piles in new locations off points and near shore that I knew would hold crappie staging to spawn or move around the lake. Good luck!