How much sugar goes into a 5 gallon batch?!
They way that you actually do it is to make a simple syrup which is equal portions of sugar and water that you melt together in a saucepan. Ta da, simple syrup.
As you are using canned fruit, I would start with maybe 1 cup of simple syrup for a 5 gallon batch. Beyond that, I would add it to taste. (More often than not a tart juice like cranberry is used [so that it looks like wine] and that requires quite a bit more simple syrup to take the tartness away.
The good thing is that simple syrup can be held under refrigeration indefinitely, so you can make a quart, use what you need and keep the leftovers covered in the refrigerator.
I hope this helps
25 lbs. ought to do you.