That's actually a good solution.

Giving them codes that relate to the total cost of their receipt, so for example 1 coffee = 30 minutes, 2 = 60 so that way when you have a group you each get 30 minutes or something.

Now there is a good startup idea... ;-)

I've seen this done in many (unmemorable) places.

for me this is the right solution.... the market wants a place to work, have internet, drink coffee, etc.

just charge for it... according to consumption (time)

That's an interesting idea but not foolproof. If you have a data plan you can just tether your phone to your laptop.

It doesn't need to be foolproof to work. Even an unenforced sign that said "wifi access limited to 60 minutes" would set expectations and deter most folks.

This happens at a highly-frequented SBux in Lisbon's Rossio train terminal. There's a code on the receipt for 45 min of usage but, the one and only time I went there, I used the net for hours and it didn't turn off. I think it's just to deter people.