My last Form 4 on a Trust took about 4.5 months.

They were down to about 3 but seem have crept back up to around 4-5.

Here are a couple good links to track "average" times for receiving the tax stamp/approved Form 4.

http://www.silencershop.com/atf-wait-times

http://www.nfatracker.com/Default.aspx