In 4.5 years of year-round commuting I haven't take a bike to the LBS for a tune-up. In the first year, I took my bike in a couple of times to have the brakes and shifting adjusted and they let me watch and ask questions about how it was done. Since then, I've been doing my own work for most things. I adjust my brakes and shifting as needed (usually just a turn of a barrel adjuster) and replace cables when a simple adjustment isn't doing it (maybe once every 6 months). I use a chain checker and replace my chain when the checker says it's 1% worn (usually around 2500 miles for me).

I think it's worth the effort to learn to do this yourself (for several reasons), but if you want to leave everything but the minor adjustment to the LBS, I'd guess that once every 2000 miles would be a good starting point unless you're having problems.