I just look at the battery indicator... or, occasionally, when the thing turns off because the battery got down to 3.2v.
Usually I swap the battery long before that, but with my 30Q's, they last so long, often on the 3rd day I'll get all the way to 3.2v.
I do however sometimes glance at the puff timer. I vape at very low wattage, so I often take very long hits, 5-6 seconds... but when I'm cooking, sometimes I don't really have time for that, so I'll turn up the wattage a smidge, so I can be satisfied with faster, shorter hits.
Andria