They last for about 3 months.

Some of this depends on how much your horse uses it, if used regularly then normally within the first week.

Yes they should be always available so the horse can self medicate.

There are no side effects and they cannot overdose, again the horse should self medicate.

Yes you can combine licks for different requirements, they cannot overdose.

Yes more than one horse can use the lick, if it is a Bespoke lick this is not advised as it will be of no benefit to another horse.

Yes they are completely safe.