My husband's stage was upgraded after surgery to T3a (due to EPE with negative margins, seminal vesicles and bladder neck). His Gleason score was unchanged at 3+4. Our surgeon said the "usual" post surgical PSA schedule is every 3 months for the first year, then every 6 months for years 2 through 4. At year 5, PSA testing becomes an annual event. Because of the EPE in my husband's case, the surgeon opted to continue PSA testing every 3 months for the first 2 years. This past March, my John "graduated" to a PSA test every 6 months.
As to how much "things" can change in a few months: in August 2009 (surgery was in March 2008), my husband's PSA, which had been at <0.1 for 17 months, bumped up to 0.1. One month later (September), his PSA was at 0.3. In October, the third PSA test (which would confirm whether or not a recurrence was in the works), came back at <0.1 and has stayed there since then. I would say that things can change quite rapidly...fortunately, in our case, all turned out well and John did not need salvage therapy. I guess you could argue that if we were on a 6 month schedule, we might never have known about the "bump" in PSA. On the other hand, if his PSA had continued trending upward, one can only wonder what it would have been had we been on a 6 month schedule.
Personally, with a Gleason 8 (though all other pathology is good), I would continue with PSA tests every 3 months for the first 2 years. However, only you and your doctor(s) can decide what works best for your situation.
BTW: Congratulations on the <0.04 - that's great news!