A 120 late is a very serious derog and will hurt for quite some time. I will let others with more experience with this comment on how long it will hurt. I would suggest starting to write GW letters ASAP.
I believe it will take a year to see any improvement, and it will get better as the years pass. I would seriously consider trying to talk them into just a 30 or 60 day late if they would, but don't hold your breath.
If it was your first late, and you have had the mortgage for a few years, then your chances improve as far as dealing with them.
We are finally homeowners!!
Closed May 5th-30 yr fixed at 5.25%.