Every time, I waited until the phone started to shut off or rapidly drain below 20% battery. Replacing the battery fixed that and brought my battery life back to original (impressive!) longevity each time. As in, I can reliably use my phone for 2 days without charging, as opposed to barely making it to midnight of the first day.

You do have to "recalibrate" the battery by fully charging it, sitting on the charger for a few hours, then fully discharging it, letting it sit dead for a few hours, and finally recharging to 100% uninterrupted afterward. Maybe if you leave that out it takes a lot longer to see the impact?