How long is your driveway? I just did a rough google earth estimate of what I know I can do with my dads DownEaster truck bed sander, and with a 2.5 yard sander (what you typically see in a 1 ton dump truck) that is slightly overloaded (rounded over), sanding heavy enough to do the job, but not overkill, I can lay down about ~2500 ft, and still have a little left in the box to do my parents short driveway in town. I can certainly expend that entire box in 2500ft if it needs more on a vacation week (I'm talking parking lot here).
ThePanzerFuhrer may have a better guesstimate than me on that though.
I forget the earlier sand conversation - are you using just screened sand or a winter sand mix (with salt mixed in). Normal screened sand will freeze solid chunks that will plug the spreader if you aren't careful. Even winter mix will freeze if you leave it uncovered or let it get wet. I would suggest not leaving sand in the spreader unless its stored inside because it can freeze in the box, and thats not a good time either. I've had to run propane torpedo heaters inside the spreader with a tarp over it to thaw a frozen box before. Good times.