If you are normal, relatively quick, walker - I would say about 15+ minutes. I've only done it once to try it. LAX says 20 minutes and that's probably some average walker. You could check your bag at United counter and then walk the sidewalk on departures level straight to TBIT and then enter security there with boarding pass or T4 which has an above ground walkway into TBIT. The post-security walk isn't quite a straight line as that. I would say 10+ minutes for that one. I think I did that once as well.