A key fob battery should last at least three years, but there are many things can affect your battery life - such as what car have, how much you use it, where you store the fob and what batteries you use. A high-quality battery should help it last a bit longer (compared to a cheap one). The bottom line is that unless it is covered by a warranty, it's cheaper to spend £10 on a decent battery than a few hundred pounds on a new keyfob (not including the price to recode it).