We do them ourselves - I didn't like paying and waiting so why not? Site survey: http://www.tamos.com/products/wifi-site-survey/
. Seems feature-rich enough, works well (i.e. good predictive results) and costs a lot less. Regular updates including Extreme APs. I'm not a huge fan of its reporting features but again, the cost makes it ok.
You should do predictive and post active if you can. If you don't have time but you have the experience, guess with some spot-checking. If you aren't comfortable with that, get a partner to do the surveys(check credentials/referrals!).
Yearly would be nice but I don't think the ROI (timewise) would be worth it. This stuff runs rock-solid and with proper setup I have little issue. The very few problems it would unearth wouldn't justify the time it took to walk the entire campus.