Old Gas Question

Technical questions, if I may.

Christmas Eve eve, I was fired from my courier job.

My Tundra, which had been driven between 150 and 350 miles a day for 250,000 miles has been driven no more than a thousand miles since.
Which leads to the question; how long can petrol (gasoline) sit in the tank before it becomes stale, rotten, yucky or whatever happens when it goes bad?
And what to do about it, or better, prevent it?

Additionally, is there any reason to change from 25,000 mile oil changes?
I use AMSOIL SAE 0W-30 Signature Series 100% Synthetic Motor Oil with the Amsoil EA oil filter, both rated for 25,000 mile/one year between changes.
Author: Shatto