professional MC mechanic since 2000 ~ I make a lot of money in the spring cleaning carbs. Bunch of variables. The main issue is varnish (gas turning solid through evaporation and oxidization), this doesn't bother gas in a can beyond getting a layer of slime/sludge in the bottom. Gums up fuel filters though. Without any kind of additive, in a cold/dark place, 4~5 months of storage isn't unreasonable. Add something that stabilizes the fuel (basically slows the oxidization process, I like Sta-Bil brand for ethanol) and you can essentially double that time. Heat and light shorten the time, so if you're somewhere hot, cut the numbers in half. Where you need to take care is with the engines, Especially if they're carburated. Just "running till empty" leaves the carb bowl about 1/3~1/2 full and speeds the process of varnishing. Best to use the drain screw on the bottom (not all carbs have them, my Honda mower doesn't) to get Everything out. Fully fill the tank to get as little airspace as possible (or fully drain the tank), and be ready with a fuel filter early the next season. FWIW, don't waste your money on higher octane fuel for yard equipment, just a waste of money (you getting pre-ignition on your mower?) AND it's a less often bought fuel so more crud in the tanks (typically). You want your fuel to be from a station with as much turn-over as possible. If possible, bought 24 hours or more after they've had a delivery (deliveries stir up the sediment and water that's found in all fuel holding tanks).