Every loan has different rules, usually FHA is the most stringent, but normally if you have at least 22% equity you should be able to drop the PMI. The newer FHA loans require PMI for the life of the loan, so you'd need to refi in that case. Refi'ing would only make sense if 1) you couldn't drop the PMI any other way, or 2) the current interest rate is significantly better than the one you have.
You can ask your agent to help you out with a market analysis so you have something in hand to show the lender. With loans, you want to be pretty conservative and know you are well within what you need to get the 22%, or whatever your lender requires.
your current situation.
My Website: http://www.wfhm/aubrey-rumberger
Best of Luck,
Aubrey Matthew Rumberger
Home Mortgage Consultant
Wells Fargo Home Mortgage
600 Bicentennial Way, Suite 200
Santa Rosa, CA 95403 (707) 521-4071