No idea how long a can lasts but I agree with Roger, punctures are either unfixable or something like a nail which let out so little air you can normally pump it up and drive home before fixing anyway. I too have a can of something in my Supersport and a few times wondered if I could retro-fit a spare wheel carrier on the back - my Caterham is for purely road use, not track. Does anyone know if this can be done?