As usual the most honest answer is, it depends.
I imagine a super precise answer, if even possible, would depend on your previous and current mileage levels, your age, experience, pace of your runs, frequency (5, 9, 13 runs a week?), workouts, general condition, talent, the weather (don't underestimate the effect of summer heat), your sleep, nutrition, et cetera ad infinitum.
Generally I'd suspect for a substantial increase in mileage, if you're not adapting at all by say 4-6 weeks maybe you've bitten off more than you can chew, or have some physical problem. If you're worn out after some weeks of consistent higher mileage, take a down week to recover and freshen up, then jump back up.
Also, jump in the occasional low-key race or do time trials periodically to see where you're at. You may be getting fitter though you don't notice so much in training, because you're training enough to tire yourself out. Take an easy day the day before so you can manage a decent effort.