With an LED light it's actually pretty hard to notice the light getting dim until it's WAY below 50%. Anyway, my EDC's these days are a Fenix L1p and a 1AAA 5mm light or coin cell light. The L1p has a NiMH rechargeable cell and I charge it up once a month or so, or if I notice the light getting dim. The 1AAA and coin lights, I change the cell if I notice significant dimming. The cells in those lights last a long time. I don't worry about whether a battery might be near depletion since I generally have a spare light (at least a coin light). I don't EDC a spare battery for the L1p, but would probably bring at least one spare along (charged NiMH or lithium) if I went on a trip.
I don't generally make any attempt to cycle cells from high-current lights to lower-current ones in hand-me-down fashion, though some CPF'ers do that (generally from incan to LED lights). I generally avoid having high-power, frequently used, non-rechargeable lights, the usual source of half-dead batteries. I have high-power rechargeables and low-power non-rechargeables, and some infrequently-used high-power non-rechargeables (TL3 incan). If I used the TL3 more, I'd convert it to 17500's (rechargeable).