Unless you do a lot of improvisation and can make a tune last for 15 minutes, a rough estimate is to count on 3 minutes per tune and make sets of 45 minutes to an hour.
Hi Audrey, Busking sounds like it could be a lot of fun, and a good way to gain experience playing out. Taking the Paris subway was always fun as there were always muscians busking. Wow, the sound really carries down in the tunnels. Once there was even a harpist, opera singer, violinist, oboeist(?sp). For me playing, I have 45 min. playing and 15 off. I’ve never had to play longer than 2hrs. though.