It depends. Some people can make one carto last over a month and some people toss them after one refill. IIRC one carto holds about 1ml of juice.
Its personal preference mixed with trial and error.
I found that after a several refillsthe carto would get gunked up and start to draw hard. That brings up the issue of cleaning. Some people boil them and claim they work like new.... but IMO cleaning cartos is a waste of time unless you are strapped for cash. They were designed to be toss sways.
The best thing to do is just keep them wet so the filler does not burn. You do not want to vape burnt filler.
I agree with everything said. I only use cartos and Boge soft tip. They are so easy
to fill and no needles or syringes needed. I get about 4 days of vapeing and the
draw gets so hard, I toss them. My juice mix is about a 70/30 mix and they might
last a little longer if I went with more PG than VG. I go thru about
2-3 ml of juice per day. If I do chocolate, then a carto is only good for about
3 days or less. Most dark juices clog us the batting much faster.
I've tried the cleaning and personally do not fill that the effort gets me that
much in return, but many clean them with success.
Keep reading and learning and watch some YouTube videos on cleaning and then
give it a try.
You can then see if it is worth it to clean them.