10mg/day prednisone (for suspected pmr). how long can i take it before the need to taper? i'd like to stop after 3 weeks. i understand it may not be long enough but steroids scare me. could i maybe take breaks between courses if i need to go back on?

Never use abbreviations in medicine.

That dose for polymyalgia rheumatica is quite low initially. You should not have any problem if you want to stop altogether after 3 weeks. If you want to play safe, you can take 10 mg, every other day after week three for another week. Just to let you know most people with polymyalgia rheumatica need to continue the corticosteroid treatment for a year or more.