By the way, there was also a former discussion about this subject, here
If I can bring my two cents to the discussion, this is my point of view and how I could use this feature.
I have 5 kids at home, including 3 teens who massively use wifi access with smartphones, laptops and ipod touch...
That is why I was pleased to find a firmware such Gargoyle, to manage a little more intelligently their Internet access.
Quotas and restrictions are great, however with Time quota it would be even better.
Regarding the need :
They actually have during school time 3 hours per day on week-end and 30 minutes during week days, and 3 hours each day during holiday time.
Sometimes some of them are looking videos, or other are playing video games or just checking mails or snapchat.
I have monitored during one month their data consumption, in the idea to create quotas and extend their restriction time, to be more flexible...
[By the way, as in France we have ISP offers with unlimited data allowed, I realized with Gargoyle that they have used a little more than 60Go for the first month...]
If I put only quotas to them, based on their data consumption, for example 700Mo per day and per teen, and extend their restriction time to 6 hours during week-end and one hour during week days, they possibly can spent the whole available hours on Internet, only surfing...
Therefore wih a time quota, we can put start and end restriction hours, but also a maximum amount of time for one day, 3 hours for example. And they won't have to be connected at the same time. It would be more flexible.
Regarding the means :
I understand the reasons of Lantis and others, that is difficult to know if an host is really using internet access.
But the easiest and first test is to know if the host is connected.
At home, wifi on the router is off when it is not used, as well as on the wifi or cable hosts : we switch it on only when we need it.
Then we can consider that when the Ethernet connexion is on, the timer can begin.
It is not finest, but for many users I think it could be good enough.