The Spyder 2 that I use has an option to be reminded about recalibration that is set by default to be 2 weeks, most write ups I've seen suggest at least once per month for amateur use and in critical professional use I wouldn't be surprised if they calibrate daily.
I'm using the Spyder 3, and it check accuracy with every boot. It also adjust according to your room brightness. Definitely not a one time operation. It's made a huge difference in my work. I use Costco for my printing. They are by far the least expensive and the print quality is excellent.
I installed their printer profiles in PS and now I can do true photo proofing. What I see is what I get in print. Paid $149 for the Spyder 3 and it was worth every cent.