Fuel stored only lasts 4-6 weeks in the tank/carburettor before degradation caused by the oxidative process takes over creating gum, resin and varnish. It has been estimated that 70% of start problems are fuel related.
Check your Honda CBF250 owner´s manual on storage of the bike and it is recommended to drain the tank if the bike is not to be run for a period. If more than one month it is recommended to drain the carburettor.