Cartos, not cartridges, are made to be disposable. But if you fill them properly and don't let them get dry you can get quite a few refills on them. I use the Ressurector cartos and they are 1.7ohm and hold alittle over 2ml of juice. I get on avg 2-4 weeks on one carto, before it is starts to get clogged. When I'm not happy with the airflow, I chuck it and use a new one. How long they last depends on alot of different factors though. Higher VG juices and thIck juices will clog cartos faster. I recommend 30% VG or less, I mostly use 80/20. Also dark and cloudy juices clog cartos faster. Thin, clearer juIces will help a carto last longer. Many dark juices contain flavor sediments and I find sediments clog cartos faster than anything.
And of course letting the carto get dry can burn the wick and filler which will give a nasty burnt taste and will ruin a carto.
They are easy to use though, and as long as you fill them correctly the first fill and make sure their topped off, you should be able to have them last awhile. My buddy uses boge 2.0ohms on an eGo and he normally gets 7-10 days before he switches to a new one.