Hello. I would like to ask, If I buy antminer, Should I mine after 2 or 3 years or it will be unacceptable?
There are many things to consider. Difficulty is the one that will make the hardware no longer profitable at some point.
I used this chart when I first started looking around. One of the miners here made it and it is very informative.
https://docs.google.com/spreadsheet/ccc?key=0AmeuPljmUNHCdEpqX2RmMDFwemJyLURVUWFtZ3J3aGc&usp=sharing#gid=15You need to understand your power consumption and cost. Cost of hardware to purchase and then factor in how the difficulty will play out. As the difficulty increases which is does ~12 days then the miners mine less btc.
Hope this helps.
Fahlcor