when the guitar starts to be really noisy, it's time to change the battery...
If you play every day 4 hours a day, you have to change the battery once a year.
With EMG's if you leave your cable plugged in it drains the battery......I never experience noise from EMG's as the battery was getting low, just no sound when they died.
On the Luke, Piezo or Silent Circuit equipped guitars the input jack also functions as a switch that connects the battery to groud (e.g. turns it on). Plugging a cable into the jack connects the circuit and if you leave the plug in the battery will drain even if you're not playing.
With my old Luke, I'd notice a real lack of output and hum when the battery died. It wasn't all that gradual, it'd just be real noticeable when it was time for a change.
With EMG's if you leave your cable plugged in it drains the battery......I never experience noise from EMG's as the battery was getting low, just no sound when they died.