FHA will charge you PMI until it has both been 5 years and you've paid the balance down to 78% of the value. For my house, it is 78% of the purchase price - my appraisal was about 20k more than the purchase price and my loan was for another 9k on top of that, so I've got a long way to go to get out of my FHA PMI. (The reason the values are all so different is because I have an FHA rehab loan.)
In short, when the balance of your mortgage is at 78% of your purchase price or your appraisal value, whichever is smaller, you can stop paying PMI.
Starting Score: EQ 583 TU04 619 EX 592 (lender pull) 2010