What size wire are you running to it and how long is the run? Does the 12-V outlet have an amp rating? What do you plan to use it for?
Of hand, I'd probably use a 15-amp fuse, but that would depend on the answers to the above questions. That assumes a relatively short run with 14-ga wire and that the outlet you have is rated for that current (I would suspect it is rated for at least that).