[AccessD] How to charge for new db features?

I'm currently developing an 'auto-installer' for one of my clients, based on
some of the ideas I've gleaned from this list.  It will automatically copy
over new versions of FE databases from a folder on the server as needed.

The client didn't exactly request that I built this, but I decided to try it
based on comments from him (I have lots of leeway about new features), and I
know he'll be happy to pay for the development time (approx 2-3 hours) when
he finds he no longer has to fuss with copying new versions over his
network.

This feature will be useful for my other clients with the same kind of
setup, and I will incorporate it into updated versions as I create them.

I'm interested to find out how others charge for this kind of development
work.  Once the code is tested, it will be 10 minutes work to insert it into
an existing FE; do you charge the development client 3 hours and everyone
else 10 minutes?  Split the development time between all the clients who can
use it?  Charge everyone 3 hours?

Thanks,

Doug Steele