Quoting LEGENDARY JAS ♥:" Actually, you want it to wear off by the time you start pushing. They can give you more but its better ... [snip!] ... its better to not be numb while you're trying to push a human being out of your crotch. Pushing is not painful, laboring is."
LOL pushing is very very painful, you obviously haven't had a baby without an epidural!
They usually give you a button to push if it starts to wear off, but it does last a long time. If its your first baby it will probably take longer to labor so you might have to push the button, just depends on how long it takes to get you to 10cm and start pushing. Just remember when you push, whether you can feel anything or not you have to push really hard from the start and you push like your having a BM so your like pushing out your butt and you have more energy when you first start than you do if your like some people and push for an hour or more.
Don't worry it will all work out! Good luck!