As we enter the lack of *blink*blink*blink* zone... LOL
I change mine when I notice a drop off in vapor (901 atty with cart) or if dripping on a 510, I notice I am hitting it harder and the atty sounds flooded even tho I am still getting decent vapor and I know it shouldn't be flooded. Since my Little chuck is new, I'm still learning its signals.
Basically, I notice I am no longer feeling satisfied with the quality of the vape. Then I just change the battery out and think... Ahhhh I love fresh batteries!
I keep 4 18350's in rotation and use 3 per day putting the third one in shortly before going to bed for the night. Per day for me = 5:30 am to about 11 pm if that helps.
On my big chuck I use one battery (18650) for 2 days pretty much but have pushed it to 3 once. 8-o