How regular are the free shuttles do you have to book

It's easy

Yep, reserving is best bet!

Every 20 minutes until 10:00, then less frequent. Book when you check in. Trip time 15 -> 20 minutes.

It's better to book the shuttle, they are 20-30 minutes apart.

I want to say from the hotel to the airport they are pretty frequent... Like every 15-20 mins or so. They do suggest you reserve a seat. From the airport to the hotel it's usually pretty regular but I wouldn't rely on it.