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I have done the trip in a scooter (the whole classic trip in fact), and it took me about 3 hours, and it certainly is possible to do the trip in a day.
But I recommend you to stay a day in Pai. It's a nice little town, and one of the best things to do it watching the sunset at Pai canyon, which you wouldn't be able to do if you were to return the bike at 6PM. There are plenty of scooter rentals in Chiang Mai, and you can easily negotiate to get the bike for 2 days which gives you a night at Pai.
When you have a scooter, the Tham Lot cave is an hour away from Pai. Self plug: https://travelemy.com/Thailand/Tham-Lot-Cave
Upvote:1
It may depend a lot on how confident you are on a scooter, and specifically on Thai roads with Thai traffic.
It's around 120km one way.
This video shows the trip and runs for about 2 hours. It's unclear exactly how close to the town it starts though.
Adventuresunsets points out it's windy, with "762 turns during the whole 3-4 hour drive." That's probably not ideal for a beginner.