There are many factors and i could literally wax intellectually here at great depth about it... but i won't. i'll keep it simple. Most board manufacturers recommend a tune-up about every 5 days of riding. Anecdotally, most New England riders seem to average about once a season (either at the beginning or the end of the season before putting their boards away for warmer weather: which is awful because with all the ice and hardpack here our boards get really thirsty really quick - much faster than out west). Our shop has a few team riders that wax after every day out.
So, somewhere between every time and once a season. If you ride boxes and rails you're going to be scrapping wax off quickly. If you ride 3 times a season and speed is not your friend, you might get away with 1 and done.
Without getting too much more into it, you should really be checking your set-up over every time you are going to go out - bolts, bindings, straps, base, and edges. If there are white areas on your base - your board is thirsting for some wax. To test your edges - run the top of your fingernail long a section. If it leaves a good line in your nail - you're good. If not - sharpen. If your board's been feeling a bit sluggish on the slope and your friends are all leaving you in the dust, it's probably a good time to get her tuned.
Get your board to us before Thursday and we'll have you ready for that weekend. All work done by hand. Wax, sharpen, and minor repairs: $30.