Caroll,

It depends on where you started, along with many other factors.  The best
numbers for IMRT are a slow, steady decline that starts leveling out over a
protracted period and flat-lines in about 2 years about the .1 mark.  You
can also expect a spike immediately afterwords.

I don't know the numbers.  My own radiation treatments are almost finished.
I have three more to go.

I asked my doctor what PSA she was looking for.  She said that I still had
a prostate and would still be producing PSA.  She said she wanted the
PSA to eventually go below 1.0 and stay there, but it could take a year or
more to settle down to that level.  She predicted that there would be
variations in the PSA because the radiation puts a lot of stress on the
healthy prostate cells, causing them to increase PSA output until
everything settles down.

I asked her if there were still live prostate cells, could they undergo the
same process to grow new tumors?

She said, No, that doesn't happen after radiation.  She implied that if the
cancer comes back it will because of existing tumor cells that were
not killed.

I trust her judgment in all this.  She's a radiation oncologist and
scientist
at the National Cancer Institute with lots of published papers on cancer
research.

So I'll be looking for 1.0.  If I don't get it in the first year, I'll be
worried.
Who wouldn't be?  But I'll continue to hope for the best and not
consider it a failure until the doctors tell me it failed.

Incidentally, if you took hormone therapy before the radiation, that might
make your early PSA values artificially low - held down by the HT.  When
the HT wears off, there might well be an upward bounce.  It complicates
the interpretation of what's happening, but that doesn't mean your
treatment failed.