Agree with FallenT. I vape tanks till almost dry.
But I notice a color change. I'll only add a bit, and stop if the remnant color is too dark. Then vape it dry.
I hate forcing myself to vape through dark tainted ejuice remnants.
So I make a habit out of cleaning or changing wicks before the visual cue. If you vape a certain flavor a lot, you can pretty much get the timing down to a science.
What's left in the tank, I unscrew the coil out, pinch the wicks (or press them against the inside rim of the tank), and pull down to squeegee it out, then pour that tank into an empty sample bottle, and store for a rainy day or when you're feeling brave. It's either that, or down the sink. Waste not want not.
That's another reason I like menthols so much. The wicks never ever need cleaning and the ejuice will never discolor. Saves money on replacement coils too.