Ages. They are caused by a virus. In my experience, over the counter remedies don't work. You'd be better off going to see your GP or a foot specialist and getting it treated with either silver nitrate or what used to be called 'drawing paste', made from magnesium sulphate. You used to be able to buy Mag Sulph paste from the chemist, but I'm not sure if it's still available and, in any case, you need to know the technique for applying it, which involves cutting a hole in a piece of plaster and sticking it over the offending verruca, dobbing on the mag sulph paste, then covering it with another plaster and leaving it for a week, during which time you can't get your foot wet.