At what voltage do you typically recharge your Li-ions?
As a poster previously said, if I take it out to check it then I'm probably going to top it up unless it's showing 3.8v or more. 4.2v is normal hot from the charger. I feel taking it down past 2.9v isn't good for battery lifespan and I would have topped it back up by then anyway. Lion lights I carry and use get checked weekly, unless I've been using it a bit. Then, I just check it after a session.
I do not charge them unattended. I like to feel the jacket to make sure it isn't overly hot. Too hot could mean the charger missed termination and it's going for deep fry of the battery. I never leave charged batteries on a charger.
Playing with the meter and comparing it to what you are using is a great way to get a feel for what's left in the battery without bothering to check. If it's been so long you forgot, time to check if it's lithium ion.
If I haven't used or checked a multicell lithium light in a long time, I do not turn it on until I have checked in case one of the cells has croaked. I'm paranoid, but I hand load firearm cartridges and have both eyes and all my fingers. I like keeping it that way. They're nice.
Anyway,
single cell lights bypass much of this fiddling about and are far, far less a cause for concern.
If my usage is very light, then I go months without actually charging. Most of my EDC is low self-discharge single-cell Eneloop NiMH for low maintenance and concern. You don't have to worry about that setup at all.
If you haven't already ordered, also consider
THIS Drok version. Few $ more, but I think it has a wider voltage range (up to 13V) for auto use, and has a clock to track output mah, for variable currents (like from solar panels).
Caught it just in time, thanks! That's even more awesome! Gotta have it. I'd have just ordered it as well, anyway.