As with pretty much all other options or variables that you can set in the program, the change is semi-permanent. It will continue to use this value until either you change it again, or you restart the program, at which point it will default back to what you have set in the plugin options.
If you are going to be frequently changing the location, it will probably cause problems with our free weather underground api-key though and the plugin may stop working for everyone, so please don't overdo it. When using a single location, the plugin will only refresh the data at most once every 15 minutes, but if you change the location it will force a refresh.
We are only allowed a total of 500 calls per day, and a maximum of 10 calls per minute, for all VoxCommando users combined.