For R, 6pm on that day would have been pushing my luck at that age. If I were feeling brave I might push it to 5.30 knowing at that age she'd tack on and we'd probably get a 6.30/7am WU but to leave it longer would make her OT and guarantee me a 5am start. So what am I saying? If you think your LO can handle it, go to 6pm, otherwise don't push it past 5.30pm
Hth x