You should probably be fine spacing them out an hour apart, but it is all going to depend on timing, crowds, ride locations, any technical malfunctions and delays, etc. If your first FP+ is located on one side of the park and the second FP+ is on the other side of the park, combined with a slow-loading ride and crowds...you just never know. I believe the rule of thumb is you actually have an hour and 20 minutes to use your FP+ (5 minutes early up to 15 minutes late), but that is at the discretion of the CMs working the ride that day. I think most FP+ lines usually don't take longer than 15-20 minutes, although with the newer popular attractions (like FoP), there can obviously be longer waits....