DL at SFO has 8 gates (40 - 48B) on the domestic terminal, assuming there is no crazy IRROPS (ATC Hold, weather, etc) and they don't dump you at International terminal, 50 min is plenty. you can check flight status on DL website to see arrival and departure gate information any day "AS A REFERENCE" to get a better understanding of what gates DL use for similar to your iten.