Prius Plugin 2015 (last year of that model) - full charge/discharge at least 3-4 times a week, currently still a bit more than 80% of capacity (granted the battery seems somewhat overbuilt, yet it is normally does 10-15C which is much tougher mode than in a pure EV where 2-3C is usually enough and only high-end Teslas and the likes would do 5-6C). There has been large continuous improvement in lithium batteries over the last couple decades.
From my model airplane experience, I believe it's "capacity per hour". So, a 1Ah battery discharged at 1c would mean 1 amp; discharged at 10c would be 10 amps. The higher the C, the harder the batteries are being used.
1 C current fully discharges battery in 1 hour. Thus 4KWh battery running 60 KW engine means 15C current, and it would discharge the battery in 4 minutes (in a very simplified linear model).
