It's got to be deep discharge. 

Let's do some math: 24V * 20A-h - 480 W-h.  A 5 S, 3300mAh pack is (5)(3.7V)(3.3A-h) = 61.05 W-h.  If the charger were perfectly efficient (they never are) and you could suck that "deep discharge" battery down to nothing each time (you shouldn't), and you sucked the flight batteries dry (you shouldn't), then you'd get almost eight charges.

Figuring on five charges for your 75% discharged packs is probably conservative, but it depends on your charger -- you'll want to check for yourself.  I'd buy a cheap voltmeter from Radio Shack to monitor the mower and just leave it connected.  Wikipedia says that the acceptable lead-acid discharge voltage is between 1.75V/cell and 1.95V/cell -- that works out to 18 to 22.8V.  I'd just keep an eye on the supply battery voltage, and if it drops below 22V or so I'd call it a day.  I think you'll get more than five charges.

Tim, I was being conservative.  Thanks for corroborating what I had crudely calculated myself.

Mike, I'm looking at chargers like the Icharger 206B which needs greater than 18v to achieve its rated output and the Thunder 620 from Hobby Partz which is pretty much a knock-off of the 206B and specs the input voltage as 10 - 28 volts.  Also, it seems that a number of people on this forum are using converted server power supplies and they are 24 volts.

Mike, I'm looking at chargers like the Icharger 206B which needs greater than 18v to achieve its rated output and the Thunder 620 from Hobby Partz which is pretty much a knock-off of the 206B and specs the input voltage as 10 - 28 volts.  Also, it seems that a number of people on this forum are using converted server power supplies and they are 24 volts.

Learning all the time.
Thanks David

Every time I read this kind of stuff I realize I don't know much at this point. So much to learn, so little time.