At our plant we meg critical motors as often as possible. Then we get whatever else we have time for. There is no good time schedule to meg motors because they can fail anytime.
I would say find your critical motors and meg those as often as possible, then pick up whatever else you can. Meg testing doesn't really tell you everything you need to know. Investing in a motor tester that does surge, hipot, step voltage, meg, and winding resistance, is the right way to do it.
We have a pretty solid PM program with two guys dedicated to motor testing, and infrared/ultrasound. These guys more than pay for themselves each year. Finding one bad motor before it fails is payback enough.