As mom2rtk said, DLR is enforcing return times, so you must plan on returning within the time frame printed on the FP. The return window for a standard attraction FP is one hour (e.g., 10:00 am to 11:00 am). There may be a grace period, but it depends on the CM on duty, the crowds, etc. You cannot count on there being a grace period (I've heard everything from 5 to 15 minutes). The only times that FPs are consistently accepted outside of the printed time window is if the attracted was shut down during your time window. At that point, the CMs will usually honor the FP later in the day.