Take a look at the demo I linked to in the OP.
You can either let it refresh every x seconds, or you can trigger ad requests with a method call when you think it's appropriate (between levels, for instance). You can also change the adbar's visibility at any time, if you need to get it out of the way so that it doesn't interfere with a particular display scenario.
eCPM will depend very much on your CTR and what kind of ads AdMob gives you (how expensive they are); in general I'm pulling in about 5-10 cents per click (this is after AdMob has already taken their 40%). Figure for your CTR to be somewhere between .3% and 3%.
Right now there are more clicks available than there are people paying for them, so you'll only get a quality fill rate for part of the day, before all the advertisers' budgets are spent for the day (at least, that's what I'm seeing, and what AdMob support more or less confirmed for me).
Keep in mind, the SDK has only been available for something like 10 days (and I personally just released my app ~40 hours ago), so all the rules of thumb are still being written.