If you're stacking testosterone (at a decent dose) with the deca, it would make sense to keep running the Propecia.

As for how long to wait, that's a tough call. The half-life of finasteride is relatively short, on the order of 4-6 hours I believe. So one would assume a few day's wait would suffice.

However, the blockage of the reductase enzyme can continue for a few months even after cessation of the inhibitor (fin or dut). This is why some people complain of side effects even months after they've stopped taking the medication.

I'm not 100% sure, but if it were me, I'd probably wait a couple weeks...