There are many factors in tire life -- tire construction and material, tire inflation, road conditions, weather, rider weight....probably in more or less that order.

I typically retire a tire when it starts getting flats frequently, which for whatever reason seems to happen well before the tread is worn out. I've generally found this to be around 2000 miles for a rear tire but as little as 1500 for a racing tire. But this is highly specific to my particular factors (as listed above). The tires I've gotten the most mileage out of are the ones I'm currently using -- 700x35 Schwalbe Marathon Supreme @ ~3100 miles and counting. They still look good and haven't flatted since mile 947 (but check with me again tomorrow now that I've said that). The tire I've gotten the least mileage out of was a 700x32 Schwalbe Marathon Supreme that suffered a fatal sidewall cut at 8 (eight!) miles.

There are a lot of trade-offs that go into tire design. A tire that is designed for high mileage usually doesn't grip the road as well and/or has higher rolling resistence and/or costs more. The trick in buying tires is to figure out what your personal priorities are and then find a tire that aligns well with those priorities. The tires you have on your Muirwoods now are a good starting point to figure out your priorities. Ride lots and take note of what you do and do not like about the tires. After 1000 miles or so you'll be ready to make an informed decision about your next tire purchase, though you may not need to make that purchase until 2000 or more miles.