If you don't ride all that hard then you should be able to get away with replacing them every year or so. You can tell when you need them when you're brakes start to require more pull to engage them, or they seem to be noticably weaker. It is VERY important to avoid any kind of oil or lubricant getting on the discs. That includes hand oil! That will seriously degrade performance so don't touch the discs. You can clean the disc itself with isopropyl alcohol and a clean cloth, but the pads will be contaminated and should be replaced.
Its a good idea to clean your discs periodically anyway. If your brakes work just fine for you then don't worry too much. Its not a bad idea to have spare pads laying around though. Have fun!