If you apply a particular medication regularly the organism at which it is
aimed will be subjected to strong selection pressure and is likely to become
resistant sooner rather than later.

Resistance comes with a cost and that cost is usually expressed in diminished
reproductive success.  That doesn't matter to the organism so long as it is
the only reproducer on the block and the medication continues to be applied
to eliminate its rivals.  However, if the medication ceases then non
resistant fellows will have a competitive advantage and will out reproduce
the resistant types.  Resistance is not only useless in the absence of the
medicine to which the organism is resistant; it is a positive disadvantage.

Thus the number of generations of the organism that is not subjected to the
medicine is important.  Alternating between two, or preferably more,
medicines and not treating at all if possible, therefore leaving long gaps
between treatments of any sort is likely to prolong the efficacy of all the
treatments longer than if used to exhaustion serially.

Resistance is always present and is never lost.  The trick is not to
encourage it, except in bees v varroa.

Chris