(1) All inquiries stay on your reports for two years, after which they fall off.
(2) The FICO models ignore all inquiries older than 365 days.
These two things are true regardless of what kind of inquiry (mortgage, auto, credit card, etc.) something is.
The following additional things are true about mortgage based inquiries (which is what you have asked about).
(1) For the first thirty days after a mortgage inquiry is conducted, FICO ignores it, meaning it has no effect on your score.
(2) If several mortgage inquiries are conducted in the same short period of time (14 days) FICO will treat them as if they were 1 inquiry.
EXAMPLE:
On April 4, you go to a mortgage lender and have them pull your scores, to give you a pre-qual estimate. Between April 7 and 16 you do this with six other lenders. By the time you get to the 7th lender you have six other mortgage inquiries on your reports, but FICO ignores them since they happened in the last 30 days, so they don't affect your score during this time.
On May 4, FICO starts to count your inquiries, since 30 days have elapsed.
By May 17 FICO is still counting all 7 inquiries is if they were one inquiry, since all 7 happened in the same short period. For the next 364 days those 7 inquiries will be treated as one.
By May 17 of the following year, all 7 inquiries will be > 365 days old, and therefore that "one" mortgage inquiry will no longer be counted.
One final way that mortgage inquiries are different from (say) credit card inquiries, is that a CC inquiry is usually run at only one bureau. When you go to a mortgage lender, however, they almost always pull all three bureaus.
