I have the smaller 3.5mL tanks and he uses the larger 6mL tanks. He will go through two tanks with different juice in them per day. I think that's a lot but he smoked about 2 pks per day, I smoked a pack. So do mine just last excessively long? Not that I would complain about that.....
How long does it take you to vape a full 3.5ml tank? If yours lasts a day or more, he's vaping about double what you are. Pair that with chain vaping causing the carto to stay at a high temp, it's no wonder yours last so much longer. I'm not saying it's dangerous or anything, just that the constant vaping combined with the constant high temp is going to cause the carto to clog much quicker. Think of it like making gravy - warming it up makes it thick. If you leave in on the warm stove for a while, it gets too thick, and if you try to add more liquid, well, it's too late.
Plus, some cartos just last longer than others, and there's no rhyme or reason to it. You may go 3 months with no problems, then all of the sudden, yours may be the problem & his will be fine......