The sooner you start your defense the better. Waiting for the "right" moment may make it so that when it comes you are in a worse position for responding.
The only problem with doing the umpa quickly is that you have to have control of an arm and a leg to make it work. If he's sitting up, he simply puts his arm out to stop himself from rolling. That's when the elbow escape works well. If he counters the elbow escape, you can simply return to the umpa.
If you do the umpa right, it's very hard for him to maintain his balance while either holding you, hitting you, or taking off clothes to rape you. It uses the strongest muscles in your body (glutes and thighs), so you have a lot of strength already there to do the move. And, the longer it takes, the less likely he'll be able to pull it off.
There are several great books on grappling and BJJ. IMO, the Gracie grappling book is terrific for learning self-defense applications over gi-oriented books. At least check them out to see the details of the moves. When practicing, it's better to find someone who's at least somewhat trained with the move (since leverage over strength is so important) rather than just anybody.
It's a fun venture to start. When I started, the school I was in had a lot of women in it. Since the women couldn't out-muscle the guys, they had to learn leverage faster and were thus able to learn a lot faster than most of the guys.
WhiteBirch