Completely depends on the project. For general, everyday work, I'll go out for a day or two once every two or three weeks. For something that needs more detail or collaboration, I'll go out for a week and then not go back for a month or five weeks. If I'm working on a project outside of my normal office (I mentioned Guam and Amsterdam, but I've had projects elsewhere, too), I'll work from home while I can and then travel there for two to three weeks and during that time I wouldn't travel to my actual office for months.