Many theories on this. Some say a year is too much, others say years are fine.
As GA says...all depends how it's stored. Also depends where it's sourced. Over here it's been proven that supermarket petrol is not only a lesser quality with less additives, but also is stored and transported in a less than adequate way, compared to premium brands.
All the above nonesense aside...it depends on how much you have...if it's only 10-20litres, from last year, is just top it up with a greater quantity of fresh fuel.
Either that or just ditch it.
Other theory is that one should either leave the fuel tank full or dead empty. Apparently avoids moisture build up.
I think this is more of a concern with diesel and not petrol.
For what it's worth, at the end of the season, I always run the tank as low as I can and have the last 20l mixed with 2 stroke at 50/1. Never had a problem in 30 odd years of ownership.
Might not answer your direct question, but hey, I'm on a role ! Haha.