If I have a group policy, and add a group/user/machine etc to that group policy, how long is it until all domain controllers have that change in effect?

This is a Windows 2003 Domain set up with controllers at different geographic locations (Each with a different L3 network).

I realize it probably depends, but how do I figure out how long it generally takes for my given setup? Also, is there an event I can check to see if it has a reached a particular domain controller?