From memory (so maybe unreliable!) 20A (amps). There are references to wiring diagrams on this site, from these you'll be able to see the rating of the fuse that protects the cigarette lighter.
If it is a 20A fuse, that means the fuse is designed to pop at about 20A, so you might want to avoid drawing any more than 15A (if you do draw 20A then the fuse might pop after a few minutes ... or an hour ... or never).