I generally go for 3 hours +/- a bit of contingency.

A little less if travelling in business or travelling from my home port.

I add a bit more contingency when downroute, especially flying out of an unfamiliar airport.

Another factor that increases required contingency is if there's no airport train, meaning you are relying on road transport to get to the airport, which could be affected by congestion. At airports like Narita or Zurich, you can pretty much set your watch off the airport train timetable.