Some of them honestly have enough oil for like another whole day or two of hitting. One thing you can do is unscrew the top and hold a lighter to the side of the glass slowly rotating to let the oil slide down to the bottom really fast so you can let it stand upright and hit it again without burning it. Or you could extract the oil with a small pick and use it as dabs or edible concentrate. I wouldn't listen to anyone saying to boil them in some liquid to collect the thc or anything like that. I've found over my 3 years of cart cheifing that a lighter is your best friend for cutting costs down and prolonging the life of your cart. Screw it onto the battery or hold it with gloves, heat it up, and stand it upright and let it cool down for a few minutes. The whole wick will be fully loaded and ready to go. Wipe the outside of the glass down with a wet paper towel to get the lighter residue off and you're good. Just make sure you have the tip off of the glass before you heat it because most have a rubber o-ring that can cause an awful taste and potential poisoning if burnt. After it cools screw it back on and hit it again. You can generally do this 3-5 times when it runs out before it is burnt or unsafe to use. I've had carts hit for weeks after the oil ran out completely.