How often is it appropriate to ask for a salary increase?

Raises should be requested every year and according to your career plan. If you are on schedule financially you should allow the employer to make the next move.
However, if you are behind the pay curve, you need to request a raise the next time you do something positive for the firm- awarded a project, completed a project under budget or before schedule, etc. Your leverage is at its peek when you have just been recognized for good performance.