How far will a wireless conection go?

Each wifi source broadcasts as far as it will go. Some go near, some go far. There is no general rule, except to try to connect. If your computer can "see" the SSID, there is a chance for a decent connection.

...just a chance.

Windows are better than walls, and Panera bread probably doesn't go out of their way to make it easy for people outside the building to get reception.

I don't like the noise level at most Starbucks, so I sit in my car if I need to connect, but sometimes I need to get just the right parking space.

I doubt that a standard laptop setup would get any farther than the building across the parking lot, and then only at a nearby window.

There are several factors other than distance which may be more important than distance. For example, what's inbetween you and the wifi site (metal? wood?), how much interference is there, what kind of antennae do you and the store have and if the antennae are directional how are they pointed, ...

Try it, assuming the store doesn't mind and you're not sending anything critical over an unencrypted network. If the signal is marginal, you might try getting a better antenna.

Better? At what, blocking the signal? UV coatings on some glass do a heckuva good job blocking a wifi signal. Not completely, but more than an interior stick-framed wall.