Welcome Benny!
I drip on a mini also known as the "super cigarette" and figured my usage at 2-3 drops to be approximately between 300-325 cigarettes per 15ml bottle. (depending on how many visits to the bar I take because obviously, I smoke more then) I drip almost every time I need a smoke and puff in between to dry out the cartridge even further. I smoke approximately 10 esmokes a day and I can make 15ml last for a month.
I used to smoke a pack of tobacco a day. I save A LOT smoking ecigs!
As far as reusing your carts I did a test. I went 40 days without doing anything. I know. But I wanted to see what would happen. Around day 40, there were signs of moisture(mold) in the mouthpiece. Around day 20 I noticed that the batting in the wick(nicotine pad) was pooling.
So... my suggestion is every week, give your wick a good fluff by using some tweezers to pull it up and rearrange it. Then at least once every 15-20 days give it a good rinse in warm water. (Pull out the wick/housing before washing.) Let dry on a towel over night. Then, when they do fall apart on you, have some new carts ready to go.