It depends on the bank. It can take up to 6 months for some of their formulas to reflect a prime rate cut (which typically takes place soon after a rate cut by the Fed.)
I have heard 6-9 months for it to trickle down to consumers in some cases. So Alan Greenspan is actually just making the banks richer for several months. Why doesn't FHA lower their damn rates quicker? I hear all this talk about "buy a car, buy a house", well, lower the mortgage rates some more, Uncle Sam.
It really depends on the bank. Some update monthly, some quarterly. My credit union is pretty quick. Chase and Citi are pretty slow.