To press/push one's luck

To press/push one's luck - to try too hard to get a particular result and risk losing what you have achieved.

Example - "She's agreed to help on Saturday, but I think I'd be pushing my luck if I asked her to be here the whole weekend"

Image - http://thumbs.dreamstime.com/t/know-limits-do-not-push-your-luck-illustration-felling-off-edge-50139984.jpg