This is almost an impossible question to answer for a plethora of reasons.
1) Juices vary. Purity of PG/VG, type and ammount of sweetner, flavor bases and ingredients, PG/VG ratios, even colorings all contribute to how much crud builds up on your coils and how quickly.
2) Vaping power. Depending on what wattage and/or voltage your device is set to, or capable of producing, produces different temperatures. The higher the power, the hotter the coil gets and the faster the wick burns up. The lower the power, the cooler the vapor, the faster crud builds up on your coil.
3) AIRFLOW! Yup, another variable to add to the equation. The more airflow the cooler the vape and the lighter the flavor. The lesser the airflow, the hotter the vape and the fuller the flavor. This has to be balanced to the power properties above, but with air flow, with too little, the vapor sits on the coils too much/too long and you get build up/scorching joose and gunked up coils...
4) SURFACE AREA. This determines the Heat Flux (mw/mm^2) which is basicaly how concentrated the heat is for a given size coil of a given ohm load of a given gauge wire fired at a certain power level.
So you can see why it is impossible to accurately answer your question...
However, for most new vapers, a coil can last between a week to two weeks. Sometimes longer, sometimes shorter.
Two things you can do to enhance the life of your coils is to:
1) Always keep plenty of joose on/in your wicks. sounds like common-sense, but we know it isn't common. The more juice in in a bottom coil style device, the less effort the wicks have to do to get fuel to the coil. just keet her topped off as much as you can.
2) PRIME YOUR NEW COIL!!! Don't just throw it in and fire away, this can either burn up your brand new wicks because there has not been enough time for joose to saturate it, or worse, the temperature gan get to high so fast because there's no joose and "pop" your new coil. (the wire gets too hot and melts) Taking a few pulls with out firing can prime your wicks by drawing fluid into them, or you can let it sit for 5-15 minutes depending on how thick your joose is.
