@squashua15
The contest was said to be over when they ran out of blocks, not after a certain time. I think Nishikata only mentioned time since he concentrated on speed for his tower, giving him the opportunity to use more blocks from the box.
He just didn’t anticipate Tagaki knowing where...
@Kaarme My guess is they are able to use lesser amounts, just the MC doesn’t know how.
After all, don’t forget all his knowledge on magic is coming from Roxanne, a non-magic user. I’m guessing there’s quite a bit she doesn’t know about it.