1. Where are you flying from?
2. What airport are you connecting at?
3. What airlines are you flying?
All of these questions have a bearing on connecting times and if it is enough. As Havermout said about London Heathrow that is a real shitty airport to connect at. But if your flying into say Frankfurt it will still be hectic but it can be done. The airport your leaving from has allot to do with it also. If it is Boston Logan, your screwed. That is one of the worst for delays leaving, as is NYC LaGuaidia, and JFK. Airline???Some have a reputation of always being late ie US AIRWAYS(or USELESS AIR) as we call them. My advise (and i kinda know what i speak of as i have flown over 1,000,000 miles overseas) 4 hours is good but 5 or 6 is better, if you can tolerate the layover. Then again if your flying American then you might get by on 2 to 3 hours layover.