I always reboot the machine when I download a new version of WeatherCat.
Me too, Blick. It's like when you move a piece of furniture, you end up sweeping and vacuuming everywhere. Or, if you're already outside and sweaty, and you see a few things you've been putting off, it's time to get after it. Heck, if you're going to have to restart WC, then give it a fresh foundation to work from.
To answer the OP's question, I don't really think about it, but sans power disruption, WC upgrade, and OS X upgrade, it can go months. That rarely happens however, due to the frequency of power disruptions, WC upgrades, and OS X upgrades (but mostly power disruptions). One time, after a prolonged power outage, I was so glad to get power back, after resetting clocks and restarting my 4 UPS's, I forgot to turn on the WC computer before going back to work/play on the main iMac. It was 2 days later before I noticed. I really should let it run a screen saver. Now, it just goes dark.
I used to be a simple man living a simple life enjoying simple things. That was before I got my first computer, though.