I wouldn't worry about it too much. Most carts that are pre-filled come sealed, and any blank carts, if you want to fill yourself (more choice of flavors, cheaper), come with an end cap to 'seal' the liquid in (except the VX2/J118 - no caps on blanks).
Depending on the model, you won't be using more than 1 or 2 carts per day, and a cart left on an e-cig (just keep it out of direct sunlight/direct heat if possible) will be fine for at least a couple of days, during which time you'll notice a lower amount of vapor and throat hit, letting you know it's time to refill or replace with a new/fresh cart.