There are a number of signals that indicate the cartomiser needs to be changed. The volume of vapour will decrease or some vapour will still be visible, but the flavour may start to taste slightly bitter.

Before you change the refill it's a good idea to check that the battery has sufficient power. Always make sure your battery is in perfect working order. An undercharged, damaged or old battery will not vaporise the refill ingredients efficiently which can lead to loss of vapour and the need to replace cartomisers more frequently.