Well, it takes about 30-35 minutes (I think, i haven't sat and watched) to regenerate a tank to continue softening water. That's no problem, unless you happen to be taking a bath at that time. Not to mention you really don't want any hard water going through your pipes. Dual tanks have valves that reach the gallons used set point, start regenerating and flip to the other tank simultaneously so you have continuous soft water.
The hardness of the water will not dictate the softener size as far as I know, that number will be used to dial in how long you can go on a tank before its 'used up' and needs to be regenerated...you need to know how much resin you have, hardness, and a couple of other variables that escape me now.
See if someone here can recommend a good company to talk to and call them. They can explain all of this better than I can probably.
The volumes you use will have some weight on the size of the softener you get. Obviously, if you don't use much water, then you don't need the 'capacity' that the Duggars would need with 19 kids or whatever the hell they are up to now and you could buy a unit more suited to YOUR needs.
[This message has been edited by jed1154 (edited 3/5/2010 9:47a).]