It would be pretty quick, probably a matter of a few months, particularly if you're on the 737. SFO 737 is very junior and almost always has unfilled seats on vacancy bids.

If you a get 737 FO assignment in another base during BI, you can bid out of that base and over to SFO whenever there's a vacancy bid (which are running monthly now). Just for example, on the vacancy bid that closed 3/15 there were over a dozen unfilled seats on SFO 737 FO so even the most junior newhire could have held it.