2 experiences of this in the last week both on BA287:

First flight arrived around 6pm. Through immigration and to the BART in about 30 minutes.
Second flight arrived a good 20-30 minutes late at 7:20pm (change of clocks meaning 1hour later). Must have arrived behind a Chinese? flight as immigration was rammed and it took a good hour to clear it. Then followed flight connections the 10 miles on foot to T3 for my AA flight - missed bag drop so had to bring my case on as hand luggage (luckily the TSA weren't bothered by the 60ish bottles of hotel shampoo etc I didn't remove from my case and made the flight with 20 mins to spare.
To cap it all, the pilot said that we were all ready and were leaving early yet definitely left 20 mins late. (could have used my Emerald status to bag a beer and some pretzels like the person in the aisle did)