When Doc arrives back in 1985 at the end of Back to the Future Part I/beginning of Part II, how long had he been time traveling? Doc has a vast knowledge of the future and even had de-aging surgery so it must have been for at least several years.
shareNah. A few months at most. The deaging didnt do a thing for him.....that was the joke..
shareWasn't the deaging thing a way of them not having to put Old Man Make-Up on Christopher Lloyd for the next two films.
In the first movie they made 1985 Doc look older, so when Marty travelled back to the 1950s, Christopher Lloyd could just play somebody his own age. When 1985 Doc became a main character for the second and third one, it would've been a pain to keep wearing the make-up, so they wrote in that he'd had a procedure that made him look '30 years younger'.