What you want is one of these.

www.ctekchargers.co.uk/mxs-3.8.php

Will keep the battery topped up and healthy.

That should be enough as long as it starts first time otherwise it will need longer to replace what you have taken out starting it. A new battery should stay charged for a month or more, if you are not using it for a long period disconnect one battery lead as most modern cars will draw some continious current.

Roughly how much should I drive my car to keep the battery in good condition?

e.g. is 15 mins once a week enough?

At this time of year you are likely to use headlamps, screen heaters and washers more than usual, and the car may have a constant drain for an immobiliser or alarm. As Skidpan says, an occasional - weekly - hour on a trickle charger is the simple solution.