While there are many factors to this (what your non-training daily routine looks like, if you're cycling calories, how much cardio you need to rely on to maintain a certain pace en route to a final level of conditioning), I think it's safe to say that anyone who has gotten used to a range of calories balanced with expenditures will notice a big difference in how they're feeling day to day when their caloric equation takes a big enough hit. This can be a substantial drop in food intake, or even just suddenly having to hit the gym 7x/week when they've gotten by with 4-5 for an extended duration up until now.
I will stress that there is a point in most preps where the scale either doesn't move, or just gets stuck plenty and you've got to rely on other indicators of steady movement, and it's at those points where I think individuals really feel the pains of the process.
For an average gym rat, 4 weeks may give you a "this isn't very fun" wake up call. Others can gut out 8-12 weeks. 16-20 week preps are tough, and grinding (even the best of them), and those last few to several weeks can push people to the point of questioning just what the hell they're even doing. I've shared some stories with close friends where I would be alone in the gym at night, pushing through those last bits of cardio, just a few weeks out from a contest, and suddenly found myself on the verge of tears just wanting it all to be over... Despite what many online experts/coaches/gurus make you believe so that you hire them, no one gets into contest winning shape by smiling and having the time of their lives every day of their preps.