4 hours free
If you have 12 hours home STBY then you have 4 hours of duty time. The calculation is as follows: up to 4 hrs at home it does not count at all. The time over 4 hours counts as half to the duty time. If you are always scheduled for 20 hrs home stby then you can run out of availabilty by the end of the year even with far less then 900 hours actual flight time.
You do 7 block hour per day, get about 12 hours duty period on such a day, after 11 days you generate maybe some overtime, then you have your 8 days off per month only and the rest days are assigned as home stby...... from 03:00 hrs to 23:hrs that gives you around 8 hrs per day and +80 per month, plus your normal duty period of 11 days per 12 hours and you have over 200 hrs per month. After 10 / 11 months you are running out. Even if you dare to take your annual leave in between............
It makes no sense to me anymore with the high Airplane utilisation to annoy the crews with long STBY´s just to have something assigned and not blanked or even another off day. Three hours before and three hours after the departure slot should do to cover the duty crew and also catch for delay´s, conduct a ferry flight or whatever operation needs that particular day.