Actually, if you take the games into consideration, there might be an explanation for Biff's time travel adventure.
Spoiler: Don't read this if you haven't played the game yet and want to play it (click to see content)
In Episode V, OUTATIME, Edna goes back in time to 1876 and accidentally sets fire to all of Hill Valley and destroys it. Marty and Doc are in Hill Valley, 1931 for an extended period of time, cornered by Officer Parker (Jennifer's Grandfather) regarding the time machine, when suddenly, the entire city and everyone in it disappears. In terms of game time, the time between Edna stealing the Delorean and Hill Valley changing is about 5-10 minutes.
In terms of Biff Tannen leaving and reappearing, that actually happens relatively quickly as well, in that Biff steals the Delorean shortly after Marty abandons his post, then returns quickly enough to not only leave the time machine where he found it and run away (or more accurately, limp to a dumpster and die), but for Marty and Doc to do a quick refuel before they take off again.
HOWEVER, what throws this off is that in movie one, it takes an entire WEEK for Marty to almost start getting erased. On the other hand, in the games, this happens a few hours after Marty's grandfather is unintentionally set up to be murdered.
So now let's get some checks based on some of the bigger time manipulations in relation to the year differences and how much time passes.
Doc ends up in the Wild West
1885 -> 1955, 70 years difference (almost EXACTLY after the Delorean disappears in movie II)
Edna destroys Hill Valley
1876 -> 1931, 55 years difference (roughly 10 minutes to adjustment)
Doc Brown dates Edna, erasing our Doc and making Citizen Brown
1931 -> 1986, 55 years (it's difficult to tell based on the flashbacks happening, but it looks roughly between 10 minutes again)
Marty sets his grandpa up for murder
1931 -> 1968 (Marty is born), 37 year difference (A few hours)
Marty makes his mom fall in love with him
1955 -> 1968, 13 year difference (A week)
I'm too lazy to calculate the formula (or see if there IS a formula), but seeing how Biff travels 60 years into the past, if we assume that history changes affect the present at variable times based on the difference in time, it wouldn't be unreasonable to roughly estimate that Marty and Doc had about 5 minutes from his departure to 2015.
