Rice paper will keep practically forever, so long as it's kept bone-dry. As for the length of time to soak, you don't need to soak it at all. Enough water to get it wet on all surfaces is enough. You moisten it, set it on your work surface, place the fillings. By the time your fillings are where you want them, the water will have soaked in and it will be soft and stretchy.
If you're REALLY good, you can actually wet a stack - the moisture from one sheet to the next will keep them all fresh while you're working, but they get very sticky and is difficult for beginners to separate them.