Is the monthly ticket valid for any number of trips to and from the destination? Is there a specific schedule (time of the day) for availing it? I shall go from Messel to Darmstadt and back, using RMV buses.
The monthly is valid for a whole month and not only between Messel and Darmstadt (in your case) but rather within a region (tariff zone) containing both cities. You can travel freely and as often as you want in this region.
To find out in which cities your ticket is valid, enter Messel and Darmstadt in the fares enquiry (the date doesn't matter) and then choose "Monatskarte". There you can pick the ticket which suits you (e. g. if you will only travel after 9 am, take the "9-Uhr-Monatskarte" which is cheaper) and finally open the "deregulated cities/municipalities" pdf file to get a list of all cities in which you can travel with your ticket.
In your case, the ticket would be valid throughout zone 4000, which is the area in the below picture bounded by red lines: