The Deadly Assassin (1976) is the first time any limitation on the process is mentioned, and it becomes a major plot point in almost all stories dealing with The Master from that point forward, including in the 1996 attempt to revive the series.
However...all of that predates the Last Great Time War.
The modern series has yet to nail down any limitation on the process, although it's more or less stated in "Let's Kill Hitler" that River has used up all her remaining regeneration energy, implying that there is some limitation.
If The Master can be entirely reincarnated (twice; once off-screen by the Time Lords in hopes he would help fight the Time War; once in "The End of Time"), then it seems feasible to assume that any prior limitations on regeneration might have been altered or even lifted entirely.
The upshot, therefore, is that @FredH's comment above is pretty much spot on: if the writers choose, then the Doctor can go on regenerating indefinitely. They could choose to explain the change as an artifact of the Time War; or something the Doctor figured out afterward without the Time Lords looking over his shoulder to stop him; or simply decide not to say anything at all, on the grounds that most of their viewership are not familiar with the Classic Series and won't necessarily care.