Hitting the g-spot

I want to try fingering my girl friend some time soon and I want to make it as pleasurable as possible for her. I've heard that I can massage her G-spot while licking her clit and that will drive her absolutely nuts with pleasure.

How do I find the g spot, apparently it's about 2 inches into the vagina and on the upper wall. Will it be obvious when I hit it, like does it feel different or will she immediately react?

Also about how long will it take for her to have an explosive orgasm after I start? 5 minutes, 10 minutes? longer?

Thanks

Also, some women can't have orgasm's "inside". In fact, the majority of women need to get off on clitoral stimulation to finish and most of the time don't orgasm inside.  The best chance of this happening is to turn her on so incredibly much that she just can't stand it, then eat her out and finger her.  She should like that.  Personally it drives me nuts when my bf does that to me, so good luck!  ^.^