Before using, prime your coil (put a few drops down the top hole) Then fill tank with juice and wait 10 minutes. Between puffs, do "primer puffs" (draw air without firing) This may all be overkill, but will keep your coil wick saturated and prevent "scorched" taste. Don't do really long puffs amd let coil cool a bit between puffs. Once again, overkill, but doing these things will double the life of your coils. I can't afford to constantly replace coils so have become OCD about this. Besides that, it depends on the juice (VG/PG ratio... how thick it is) sweetness and some flavors. I vape about 4 ml a day (doing MTL) and get about a week or two from a coil. Don't push the coils beyond the rated wattage! Personally, I like 65VG/35PG juices but 70VG30PG is fine. 80VG/20PG is a bit thick and works better with different coils/tanks.
BTW, I have an Innokin T20 S tank and don't like it because of not enough airflow... I had to drill the holes bigger. Once I did this, I liked the coils much better. I use the 0.8 ohm most.