As others have noted, acceptable minimum connection times from domestic T2 to Intl. A or G vary depending on where you are coming from, weather (usually fog) conditions at SFO, and your capacity to deal with delay-related stress.

If you are coming from a West Coast destination, there's a higher probability of delays due to low visibility at SFO. They'll hold those flights first and preferentially land the intl. widebodies and transcons that have been in the air longer. So you might want to give yourself more time in that situation.

If you are confident everything will go smoothly, a 90-minute connection is likely reasonable (in the event of TSA delays, assuming you might want to get to the United gate when boarding begins rather than at the last minute, that you might buy a magazine at the airport, etc.). On the other hand, if you have a high capacity for stress and uncertainty, why not roll the dice with a 1-hour connection?

My tendency, on what sound like they are different tickets on different carriers (and assuming missing the United flight would be problematic), would be to err on the side of allowing more time, like 3 or 4 hours, and then if everything goes smoothly, have a nice meal or check out some of the airport exhibits prior to my next flight.