the answers to that question are hugely variable and depend on the objective and defining waste. if one values learning intrinsically, like most of us here probably do, it is pretty hard to come up with a waste of time, even taking the rare break from learning.

But it seems self-evident where constraints like markets or material conditions might demarcate usefulness and waste.

Even the learners who are as happy to hear about linguistics as they are material science I presume do some opportunity cost analysis as they learn. Personally speaking, I rarely, if ever, feel like I'm wasting time per se but I always recognize and am conscious of the other things I could be doing to better maximize alternative objectives. That omnipresent consciousness may just be anxiety though I guess...