Paul Andrews at the Seattle Times wrote a nice column looking at how cafes in Seattle and San Francisco handle the potential for Wi-Fi users to over stay their welcome: In Seattle, the cafes offering free Wi-Fi have pretty laid back rules. One restaurant asks visitors to buy something but has no time or spending limits. Another that often gets crowded asks other patrons who may be looking for a seat to politely ask laptop users how long they'll be around. None of the Seattle venues he spoke with say it's an issue.
In San Francisco, where Andrews believes more cafes charge a fee for access, some venues have methods for cutting down on use. One turns off the network at 6 p.m. Others have signs on tables asking users to stay for just an hour.
Techdirt notes that there's no evidence such measures are necessary. Most Wi-Fi users give up their seats if the place gets crowded plus they tend to visit during off hours when a venue is looking for customers.