FlyerTalk Evangelist
Join Date: Dec 2002
Location: Danville, CA, USA;
Programs: UA 1MM, WN CP, Marriott LT Plat, Hilton Gold, IC Plat
Posts: 15,715
To be safe, allow one hour from downtown SF until the time you are at the check-in counter. The BART ride is 35 min from Montgomery Street, plus waiting time from for the SFO airtrain. Trust me, you don't want to walk from BART (which leaves you off at the international terminal) to the UA terminal. Fortunately the airtrain is not more than 100 feet from the BART exit.
Did the whole thing a couple of weeks ago - worked like a charm. Much nicer than Chicago where you have to walk almost a mile to get to the El.