How to tell when your cartridge is empty??

[cooperidol]

Just as the title suggests, im always unsure as to weather my cartridge is empty... im new to vaping so im unsure as to how long a filled one should last.. on my Riva 510 i put about 10 drops in a cartridge and it seems to last a forever, and i end up just changing it because "the liquid must be gone by now"...
basically is there any sure way of telling that all the e-liquid has been vaped and its time for a top up?

 

[Sugar_and_Spice]

Oh yeah....when it tastes burnt and very nasty. And welcome to ecf. When vapor productions start to go down,,,,,time to top up.

sas

 

[slimest]

If you want to cough, you should refill or replace cartridge. With normal liquid quantity at the heater no cough at all.

 

[Nomoreash]

Usually the flavor will start to drop off, if you go past that you'll get to very little to no flavor and a burnt nasty taste. If unsure ALWAYS error on the side of topping it off to often. Once you get an idea of how many drops it takes to top it off at certain intervals you'll figure out how much is needed and you WILL learn to detect when the flavor is dropping off to avoid that burned taste.