How much time do reserve officers spend working between drills?

As the good CPT stated it really depends, for some officers and NCO's it depends on your position, the unit, it's mission. I know my last unit was a 11 man liaison team and our CPT who was in the training slot probably spent about 10 hours per week, not sure what our LTC put in as the commander he lived in West Virginia which was about 10 hour drive.

It depends on your position. As company commander, there were two additional UTA's available for me every month for administrative purposes, but the reality of it always seemed that you will put in an additional 40 hours between drills. As S1, I averaged less, but still usually spent an additional 30 hours, plus or minus, on paperwork and additional duties. Platoon leader doesn't require that much extra time unless you are getting ready to deploy.