When should I kiss her?

Touches come before kisses! Would you fondle her breast before kissing? of course not! If you just grab and kiss- you could get rejected, and not neccessarily because she didnt want the kiss. She needs to have got to the point of wanting you and may just not be ready. Kissing a girl who isnt ready can ruin things, so do other things to make known your intentions first - like holding her coat for her to put on or removing some fluff or crumbs from her clothing. These things should be done casually, not with any intensity. If these things feel comfortable for you both, you can put your hand on her waist for a moment or two, perhaps under the pretence of getting past her to get a drink or something, this could be a two handed operation. If she shows signs of pulling away or being uncomfortable at your touch, then a kiss would not be appropriate YET. Also, to really wow her- the kissing should start with gentle kisses, to the side of the lips.