Seriously, how long does it really take to learn salsa?

All your life.

Ok, most people do a double-take and ask me to stop being sarcastic, but it's true. Salsa is such a convoluted, amalgamated fusion of dance styles that no can claim to truly know salsa. There's new styles and moves being introduced each year.
Realistically, a beginner can pick up the basic steps and learn a few routines in a month or two (if he/she has good rythm). I call my three+ month students Intermediates, but I myself am not at an advanced level... yet! Sometimes, people join my class expecting to learn salsa in four Sundays, but walk away thinking it's too tough. If you are interested in learning, prepare yourself for the long haul - four classes won't even scrape the icing of this fantastic dance.

Don't ever join a Salsa class if the person teaching promises that you will learn most of the moves in a few months. It's a slow, stretched-out process that can be totally fun if you start with realistic expectations.