Art fairs typically last around five days, with most of the sales happening day one (or even before, if a gallery presells part of the booth). At a fair, you can simply walk up to a gallery, ask about the pricing and availability of its artworks, and buy the pieces you want. Sometimes, you can even take the art home with you that same day.
For this reason, art fairs are great for buying art in a time crunch, as the purchasing process will move at a faster pace. If you need more time to make a decision, you can always delay the sale by asking to put the work on hold.