Mike G. What airline are you speaking.? Duty timeis continous from scheduled check in to usually 15 minutes after for getting passengers off and debriefing for both cockpit crew and FAs. Also minimum number of FAs required is dependent on capacity of air plane not number of passengers on A/C. 1 FA for each 50 passenger seats. So 51 sesats 2 FAs required. FAA requirements. Of course can have more FAs assigned especially for meal service.
Actual flight attendant speaking here working off my contract. Duty time starts 15 minutes prior to boarding which given larger airports, and the fact KCM randoms almost 2 out of 3 people these days you could argue should be 30 minutes to 45 minutes prior depending on how long it takes to get to the gate. The FAA minimum of FAs is completely dependent on capacity as you have stated. The FAR is 1 FA per 50 Seats not PAX. And god trying to match it with loads I could see that creating some problems for crew planning and crew scheduling. Now you have the scope clause with most regional airlines that prevent them from flying larger A/C like the E190 because of the agreements with the various pilot unions at the mainline carrier.
Duty off is 15 minutes after block in. Now we get to the fun things in my contract. I am allowed to work 14 hours before I go on law to borrow the railroad terminal. Now in IROPS they can extend me to 15 hours. And if the door is closed before I law out then I can continue onto the destination. This sometimes makes a good threat when I'm close to lawing out and passengers take forever to board and get settled. I've hit a few 16 hour days and they are painful but doable.
I'm also allowed to work for 6 days straight before I must take a day off. I block my days together in six day pairs so I can spend time with Viktoria for a week every month in St. Petersburg. Now that I have a line it will be much easier to do that and control my life.
Now this is an FAA thing on flight attendants other countries might do it differently but the rule is pretty standard. Some airlines especially the more service oriented ones like Turkish, Aeroflot, Qatar, Emirates, etc.. they tend to have more flight attendants. Last time I did the Turkish A321 it had I want to say 5 or 6 FAs, AA only runs with 4. Same aircraft type different airlines but they have vastly different service standards. Turkish will give you a full meal on a two hour flight. I should know I ZED on them a lot.