As you can imagine, most Best Actor/Actresses make their appearances very early in their films.
The latest entry I've found so far is Ron Moody as Fagin in Carol Reed's Oliver! He doesn't appear until 36:54 into the film (and then proceeds to steal the show.)
Caveat #1: I've only looked at Actors, not Actresses so far.
Caveat #2: I've checked out every nomination since 1953, and a good number of the ones before. But there are roughly 60 movies, all older, which are either impossible or very hard to find for online viewing or rental. Based on the plot descriptions, there are only a handful where I feel the actor might make a later appearance, and none where I feel they wouldn't appear before 37 minutes.
Some other late entries I've noted:
- Montgomery Clift, The Search, 1948 - 34:49
- Humphrey Bogart, The Caine Mutiny, 1954 - 26:38
- Frederic March, A Star Is Born, 1937 - 26:14
- Dustin Hoffman, Rain Man, 1988 - 18:32
- Robin Williams, The Fisher King, 1991 - 18:22