Games : How much time do I have before killing someone else to do a combo?

In the new Unreal Tournament, when I kill 2-3 guys in a row in a few seconds, I get a double/triple/multikill combo.

I wonder how much time I have before killing someone else after I kill someone to do a combo.

I know that it's not forever as sometimes people are scrambled across the map and it doesn't count it as a kill when I kill them.