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I just checked and we didn't actually book ferry in the end, we just rolled up and bought tickets (allowing a buffer of time before departure, I think they make you get there at least 15 mins before sailing) - but that was back in October, on the 7am crossing. Obvs now it's peak season they might be slightly busier.
We bought a return for a specific time and then missed it, but they let us on the next one anyway. YMMV - it perhaps felt like we'd been 'let off' rather than it maybe being official policy...
From memory it was about 100km around the island and we stopped in Ryde and Ventnor for something to eat, so 5-6+ hours would do it if you're not in a rush, a bit less if you're cracking on and stopping less.
I do indeed. Did it as a 400 last year.
Mostly chilled route, though Portsmouth is not hugely fun for riding, depending on when you arrive there.
I'd try to avoid rush hour if you can, which is when we arrived having set off at midnight.
Other tips: check the ferry times, and then check them again. they throw in a little gap at random points in the day to keep you on your toes, which meant we had to hang around the terminal for an hour or so.
https://ridewithgps.com/routes/34415427