Even better: have the code last longer depending on the amount you spend. Buying a high-margin drink? Have 3 hours on the house! Grabbing the cheapest option on the board? Sorry pal, it's 30 mins for you. There's a business model there, I think.
(The only problem for me is that I hate hate hate wifi portals. They all suck.)
A coffee shop I used to frequent [1] tried to take a "soft" approach to that, by having a sign near the electric outlets under the tables, asking patrons who were going to stay longer on their laptops to spend at least $n/hr (I don't remember what $n was). So people who didn't use the outlets didn't get bothered, and those who did got some guilt pressure to either buy things or not stay all day. I don't have any inside information on how well it worked, though. And this was a few years ago, so maybe that particular trick wouldn't work as well in these days of 10-hour MacBook Air batteries.
From chatting with a few managers in the Santa Cruz area (not sure how representative a market it is), they didn't want to actually drive away the laptop crowd, because that provided the majority of their business for much of the day. To some extent they wanted to be a coworking type of space, because there's no other way to fill up a coffee shop in Santa Cruz or Capitola at 11am on a Tuesday in November. They just wanted the laptoppers to either keep buying stuff periodically, or else keep their visits somewhat reasonable (i.e. not 8 hours on one coffee).
A different place I used to work at [2] actually liked me staying around all day, whether I bought something or not, on days when it was mostly empty, because it helped give the impression that the place wasn't deserted. They did most of their turnover on tourists, though, so there was sort of an informal agreement that laptopping regulars should move along when the place was full (mostly weekends or summer afternoon/evenings).
That would be OK if most high-margin drinks weren't packed with sugar and all kinds of other shit. Why punish people who want a simple Americano by forcing them to get up every 30 minutes and make a new purchase? :)
edit: Of course, the appropriate response to this is that getting up every 30 minutes to walk around and stretch is good for health. Yes, I do debate with myself in my head...
Buy four Americano's at a time, get two hours wi-fi and tell the barista to give the three extra drinks to their next three customers. You get your wi-fi, limit your caffeine intake, and help the shop generate goodwill with its customers, which in turn generates goodwill of the shop towards you.
If you're gonna be a hobo, at least try and be classy about it.
(The only problem for me is that I hate hate hate wifi portals. They all suck.)