The advantage of FP is that it essentailly lets you "wait" in two lines at once: the FP attraction and whatever ride you go on while waiting for your FP return time.
Your FP will tell you when you can get your next FP. Usually it is 5 minutes after your return window begins or about 2 hours after you got the FP, whichever comes first. However, the time can vary depending on crowd levels and the FP attraction. It could be less than 45 minutes so check your ticket.
Get a second FP ticket as soon as you can--before using your first FP. This starts the clock running on your second FP while you are going on the ride for the first FP. It is a great way to maximize it's use. It may take some criss-crossing of the park to get the FPs you want. However, only one person in your party needs to get the FPs. He/She just has to collect everyone's admission ticket, take them to the FP machine and insert each ticket into the machine. The rest of the group can do soemthing else like: take a bathroom break, shop, eat, rest, wait in line for another attraction.
Don't make the classic FP mistake. Many people see the wait in the standby line is 45 minutes but the FP return time is 2 hours away. They assume they are better off waiting in the standby line because they will get on the ride sooner. While this is true, they do not understand the concept of FP. Instead of waiting in the standby ;ine, you can be going on other attractions. During that 45 minutes they are waitning in line, a FP person could go on one or two other non-FP attractions or get some lunch, shop, etc. It allows you to do other things instead of waiting in line.
Hope this helps.