It depends on the loan program. For Owner-Occupied Lite Doc and Bank Statement programs, see below:

LTVMin. Reserve Requirement
Less than or equal to 65% LTV0 months PITI(A)
Greater Than 65% LTV3 months PITI(A)
Borrowers who own other financed properties are required to evidence an additional 1 month of PITIA reserves for the subject property for each additional financed property, capped at a maximum total of 18 months PITIA reserves.