Its 4 hours, but of you are connecting from domestic they don't make an issue. In June I went thru security at 8.30-9pm for a 1.50am flight. Even though I walked the entire terminal looking at all lounges before settling into the LH lounge it got booooring very fast. I would say you need 60 minutes to try out the AI, SQ, LH lounge(s) and then there is nothing to do.
