Sorry to duck the question, but how long is a piece of string..
Ask yourself how long does my 1700 last, then approx double it. From my car days, with the kids, I found that the batts go 'off' quite quickly, that is when they are getting on a bit they don't last as long or give as good a punch, and take longer to charge. In the end I lived with a stack of cheap 1700's and just kept swapping them around. If you get a car based fast charger, and around 4 of the 1700/2000, then you could keep going for ages, charging one set whilst running another.
Or that's my master plan!

Yours Simon M.