Topic of the day: Coils!
So much variation in results, and it takes time to understand it all. More complicated than burning a stick of tobacco.
Don't give up though. After a bit, you'll either "get into it" and study up, or just "find something that works" and go with it. Either approach is fine.
But from our end, explaining it gets to be tough. So I'll try both ways: Summary and detail. I don't know what device you are using though, so I'll just guess an eGo. If you have VV or some other adjustable mod, it gets even longer to explain.
A) The summary:
If you have an eGo and an EVOD setup, and you're vaping at about 3.3/3.4 volts (that's a standard eGo)...you want about 1.5 to 1.8 ohms. Can go as high as two ohms. This is single coil. For dual coils, you don't want to use a standard eGo. You need a mod or at least a device that can put out twice the watts because you're powering two coils (more on that later).
B) The details:
Ohms = resistance. Electrical Friction. There's a ton of posts here on ECF...do a search. Basically, the more ohms, the more friction so LESS power gets drained per second from the battery. AKA ...it's cooler as the number gets higher.
However, since dual coils have two coils side-by-side, they have 1/2 the friction (assuming they are identical ohms each). Think sucking a thick milkshake through two straws rather than one....more per second. So a dual coil has 1/2 the ohms of a single coil for two of the same coils. E.g. Two 2.0 ohm coils side-by-side are a 1.0 ohm dual coil. You use the voltage for 2.0 ohm coils, but only have 1.0 ohm of resistance (because there's two coils) and thus need twice the watts to power the thing.
So watts are about voltage (the "push") and ohms (the resistance). There's always amps too (the flow measurement).
Shoot for about 7-8 watts for a
standard single coil. Watts = voltage x voltage / ohms.
For your theoretical eGo device, 3.4 volts at 1.8 ohms = 3.4 x 3.4 /1.8 = 6.42 watts. At 1.5 ohms it is 7.7 watts.
Once you get into other types of coils, it varies again. People using rebuildable systems use other gauge wires and it makes a difference in how you calc stuff.
How long do they last??? Depends!
If you have a wicking issue and over vape them, they get too hot and pop.
If you over volt them, they get too hot and pop.
If you dry-burn them and over stress them, they get too hot and pop.
If the wick is gunked up, that's a wicking issue. Some juices gunk faster than others. Dark juices in particular.
I get about 3 weeks from one EVOD coil. YMMV.