PHB Page 119, Wizard, School of Transmutation, Minor Alchemy:
For each 10 minutes you spend performing the procedure, you can transform up to 1 cubic foot of material. After 1 hour, or until you lose your concentration (as if you were concentrating on a spell), the material reverts to its original substance.
There don't seem to be time limits on how long one can concentrate, as time is not listed in the things that could break concentration on page 203 of the PHB.
Given this, could a wizard of the school of transmutation spend 12 hours converting 72 cubic feet of material?
Could a wizard spend 30 hours converting 180 cubic feet of material?
Best Answer
It seems based on a close reading, that there is no volume limit on how much material you can transmute. You are limited by only three factors:
As long as you are dealing with a single object, made of a single material and you can keep up the procedure, there is no limit to the volume of the object you are transmuting.
Basically you have two possible readings of this description. Either:
I think that if the first reading was intended, then the description would include something along the lines of "you can transmute the material at a rate of 10 minutes per cubic foot." I also don't think the rules would be limited to a single object then, but would rather encompass 1 cubic foot of material every 10 minutes, be that 1 object such as a box, or 100 copper coins.
Further, the idea that you can have, say, a "wood" box where half of the box is wood and half of the box is stone, doesn't make much sense. A strict reading of the rules would also make this usage impossible, since once you have transmuted the first cubic foot of material, then the box would be made of two materials and you would no longer be able to perform the procedure!