You would have to ask the person whoms town you are doing the charm in. They should know. Right now, my town charm is if you catch a migratory locust in my town, you are to have bee free trees in your town.
Now I did have someone complete my town charm this month so I don't know if it's only good for one person or anyone who catches a migratory locust in my town.
When I went to Katrina for the charm, she never said what happens with the person who completes the charm.. at least... I don't think. I'll pay more attention next time