No one can give you an answer to this. Everyone's cancer is individual. There's people even on here that were given a poor prognosis as far as the passage of time goes, yet they're still here months and even years later. Those treating your mum would be better placed to give you an estimate because they're directly involved in her care, but even those prognoses will be guesses.

You're maybe better making those visit arrangements sooner rather than later just in case, because whilst it's cliché, put something off, you might live to regret it.