Depends on you loan type and when the loan was taking out (rules always changing)
just got a FHA loan in June. Got a letter a the first of the year (required by law it said) that states that i must keep PMI for a minimum of 5 years, that it will automatically be removed once my scheduled payment hit the 78% threshold (as long as ii have good payment status) and that if i pay extra payments and hit the 78% threshold it was my responsibility to ask for the insurance to stop.
Also, getting an appraisal showing the value has increased no longer is valid for helping to remove PMI insurance.
Typically it is 80% and it is based on the original selling price of the house. For example, if you buy a 100,000 house, you would need to put down 20,000 in order to avoid paying PMI. If you put down 10,000 that would be 10% equity, not 10,000/90,000 amount of the loan, or 11.1%.
Whether or not you pay PMI is based on the original SALES price of the home.
Now, the AMOUNT of PMI you pay per month is based on the amount of the loan.