Just bought 2011 with 540 miles. Going to change oil since it's probably original. Common sense tells me to change the o-rings since they more than likely have flattened.

Going forward, how many oil changes are the o-rings good for?

You could try just flipping it over ,if it isn't brittle ,the O-ring tends to flatten on the side facing the motor ,but the outside tends to remain rounded sitting in the groove of the cover.( I may be on my second one in 24 years). If it leaks( tighten the screws slowly and evenly) you'll soon see oil running down the side case, as it sees the full pressure of the oil pump ,about 55 psi at high RPM's.