We did a similar dance for the Coin Street Festival and with three bands it went on for the whole afternoon. It was a mixture of Audience Dancing and dance demonstrations and included Hammersmith Morris.
How it will work is that you get a constant flow of people watching and a few participating if you can overcome their shyness.A useful trick is to get the show dancers to go into the audience and pick a partner.
The Audience will stay and watch for about thirty minutes and then move on and you just repeat it with the next batch. From memory there were about thirty thousand people moving through at the time of the dancing.
I would suggest as it is a smaller affair to go for 40 -60 minute X 2 spots including the demonstrations.
Al