The break in point for mining now is pretty well 1THs...anything less than that is a complete waste of time and money.
Is there any way to tell how long that will last or is it just a crapshoot? In other words, if I had a 1TH miner today it would still be profitable, but could this not be the case in say a month from now?
It will changes because the difficulty will go up and more hash power is being added. Its hard to 'predict'.