The inspection window is (around) every 26,000 miles.
Yes, it is possible to do it yourself. You need to be halfway decent with a wrench and math. If you do the job yourself it will take about 2 - 2 1/2 hours by yourself the first time to disassemble and measure the clearances. Add another hour onto that if you need to change out one or more of the shims. If you're lucky, all of your clearances will be in spec and you won't have to change anything out. The biggest part of this job is patience and attention to detail. Take it slow, don't rush it.
DON'T WASTE YOUR MONEY AT THE DEALER. For the sake of your engine, and your wallet.