We were somewhere around Barstow, on the edge of the desert, when the
drugs began to take hold. I remember Dave
saying something like:

I am going to take one of our two cars off the road at the end of this
month. I had thought of running a mains extension under the bonnet and
leaving a charger there. The extension will be plugged into a timer to
control how long the chargers is on.

How long a time should I power up the charger, per day/week?

Half an hour per day would be fine. That's exactly what I do on one of
mine. So far, that particular battery's remained good for a couple of
years after being laid up.
An alternative is a decent sized solar panel inside the windscreen and
charging either through the lighter socket or via flyleads to the
disconnected battery. Again, another of my vehicles has just that.

All depends on the battery's self-discharge rate and of course if
anything's slightly draining it on the vehicle.