I tend to replace the coil in my vaporizer after every 2 bottles. Never had any problems with the coils. They've been working great as long as I don't overuse them.
Every 2 bottles of fluid I replace the coil in my Endura T18. Never had any problems with it or any replacement coil. Both vaporizer and coils have always worked beautifully.
At least 2 out of the pack of 5 will not last more than 2 days