What is a "safe" poly count when making terrain?

[CaptainCrazy]

So i've started to make terrain/displacement and i've put a few into Radiant, walked around on them in-game, etc.  But i'd like to know what is a safe amount of poly's to use when making terrain?

 

Obviously i don't want to lag things out, i mean i didn't witness any lag when walking around on my map but that's because the terrain was the only thing on it pretty much.

 

The reason i ask is because i may wish to make some semi-outdoors maps with a few hills or mountains and such so i'd like to refine the process a little and find an equal balance.

 

Thanks!