People experience huge differences in their equipment performance because that is vaping - there is no 'normal'. For some people, a coil lasts 3 days and it's done - clean it, rebuild or chuck it. Others get months out of a coil.
It all depends on so many variables that no single answer is valid: volume of liquid used, amount of flavouring (if any), type of flavouring, voltage / power used, style of vaping, etc etc.
The only thing that works is: does the coil still work - does it taste OK and produce good vapour? If not, it's time for a clean / rebuild / replace, depending on the type of coil and your preference.
I don't think there are any significant safety concerns with coils of normal resistance (= over 1 ohm) not working, or breaking. They either work properly or they don't. If you are talking about sub-ohm coils, then they are all replaceable / rebuildable - so if it doesn't work, you replace or rebuild it.
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OK there also super-misers like tj99959 above who makes a Kayfun coil last a year. Now that might be a world record, so: