I quit my job of 4 years a few months ago, and while interviewing for a startup in Shoreditch, I was preparing for the interview in a coffee shop around the corner and I hadn't banked on there being a time limit for the WiFi. I didn't think places did this anymore so naturally I was caught by surprise.
After an hour a popup appeared on my screen, right before I needed to run a web app; what is this, 2010? Dead in my tracks, I remembered a snippet of code I saved to Google keep, lucky for me it works offline and had my notes saved.
Maybe I didn't need to be cautious but I noted down my real WiFi mac address in case I needed to change it back immediately; you know how computers do unexpected things some times.
ifconfig en0 | grep ether
Now to actually change your mac address to something different
ifconfig en0 ether aa:bb:cc:dd:ee:ff
Hopefully no one is like you and using the same mac address. You could check the local arp tables on your mac before your WiFi time is up to see if someone has it already.
arp -a
Note: this is a bit unreliable because you're relying on your local network being chatty and asking for what IP has what mac address.
Now your network should reset, the coffee shop WiFi doesn't know any better, and you have WiFi again! Coffee shops do this so that they have a steady rotation of customers, I get that. But this place was empty though so I don't feel bad.
It's true that modern phones and other devices have started to randomise mac addresses now; this is to hide the profile of your device for security, so maybe turning your WiFi off and on again is enough to get free WiFi for another session.