If we have a consultans who is working for us and receives a per diem, is there a point in time that we no longer need to give him a per diem, for example if his assignment with us goes past 1 year, or 2 years, etc. Do we just keep paying him a per diem for as long as he works for us away from his home?

As far as I know there is no limit to the per diem. There are truck drivers who work for years for companies and still collect the per diem. It's part of their "pay" package for being away from home.

Not unless you have a written contract with this person that states the per diem is only for such and such period.