Some coffee shops place time limitations on their WiFi. For example, I recently went to a Panera that had a 30 minute time limit on their WiFi during lunch hours.
Getting around such limits isn't difficult. I'm not sure how ethical it is to do so, so consider this all merely educational information.
It seems these kinds of systems track you based on your MAC address. If your MAC address changes, the system thinks of you as a new user. Changing your MAC address is easy enough (on Linux):
Run ifconfig and look for the one that looks like right. Mine was wlan0.
Change your MAC address
ifconfig wlan0 down ifconfig wlan0 hw ether a1:b2:c3:d4:e5:f6 ifconfig wlan0 up
For ease of changing, you can make a script that looks something like:
#!/bin/bash
ifconfig wlan0 down
ifconfig wlan0 hw ether $1
ifconfig wlan0 up
And then run it like ./change_mac.sh a1:b2:c3:d4:e5:f6.
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