this chart will simplify for you, without doing "math". Green is good, red is too hot, yellow too cool.
on the left, your clearo at 2.2ohm, on top of chart, what your batt is set at. With a new coil, that may be a little "hot" (you are going into red there) and you should be getting MORE than enough vapor at that setting. Since you are using store bought coils I'd say maybe you got a bad one.
Make sure it is screwed in tightly. Often you will notice when opening it up that your coil isn't tight inside the clearo.
There is no way to say how long a coil will last, it depends on what you are
vaping (dark juice that is sweet is harder on coils, etc.) 3 days to 3 weeks.
This chart is only applicable for single coils, which a mini protank 2 is.
quite frankly, with a simple set up like yours, you should just "dial in" whatever tastes good to you. Since you are not using a mechanical mod the battery you have has protections, it's going to either shut down or you will notice either a too cool or too hot vape just by taste.
2.2ohm coil vaping at 4.25 volts on your batt should give a very nice vape at 8.21 watts, do you see that?
Again, maybe either the coil is bad, is dying, not screwed in tightly, OR the coil is actually 2.5ohm or higher and mismarked. . at 2.2ohms I never turn my batt up past 4.2, but that is me.
General Safe Vaping Power | Voltage | Resistance | Ohm Chart
here's a little different one, same idea