Motorcycles

The biggest factor will probably be what temperatures the bike will go through as it's parked and how humid the environment is.

The problem there is the expansion and contraction of the fuel vapour in the tank with temperature cycles. When its hot enough, the fuel vapour will be released through the breather.

If it's parked up for really long (years), the fuel will turn into a gummy substance.

The bigger issue, particularly with Ethanol in fuel is that during cooling cycles, air from the atmosphere gets into the tank. That air contains Oxygen which does some degradation, but more importantly it also bring in humidity.

Ethanol is hydrophilic, and will absorb moisture from the air. That can later separate again and sit at the bottom of the tank, causing rust.

What does all that mean?

If if the bike is parked in cold location with steady temperatures and dry air, you can probably park it for years without any issues.

If it's a black bike that you park in the sun in a location with high humidity (Darwin Australia or Florida), then then fuel will probably degrade in a couple months.