The way I do it is the following: if there is no new plane around, I would go to the plane with most hours available and go to the flight list, there I would check how many hours per week are being used and how many flights I can put for this aircraft based on the R/T blocktime for this specific route.
Explaining more: The way I utilize my planes is that I "set" each day to have a maximum of 16 hours of flight, so I go summing up the time for each frequency. For example, after I sum up all the hours in all frequencies for a plane I end up just using it for 6 days, then I could add 16 more hours with a route. Modifications to this are linked to a daily utilization close to 16 hours but that would count for a full day, like summing up and coming up with 12 to 14 hours and the sum of the next flight would pass the 16 hour threshold. Consequences to this are changing flights for other aircraft if there are more flights in the aircraft the is supposed to or adding flights to fill the hours available.
I implemented this system recently and I really liked it. Since I have a fleet of 43 planes It wasn't such a pain to review them all, but if you have more than this I wouldn't recommend, mainly if you use a lot of low to mid range aircraft because it takes more time to review due to the shear amount of frequencies per plane. Any questions, just ask.