According to BP:
Petrol in equipment tanks - The storage life of petrol in equipment fuel tanks is one month. This can be extended by topping up with one third of fresh fuel, which restores the volatile components that have evaporated. Topping up with fresh fuel will help, however it is not a foolproof strategy for engines that are used only intermittently.
The following principles may also help:
- Keep the tank half full to stop water vapour from being sucked in and condensing.
- Consider using a fuel that contains anti-oxidants, metal deactivators and corrosion inhibitors to protect metal surfaces such as BP Ultimate.