When my kids were using pacifiers I would wash the pacifiers everyday.
I would have more than one pacifier on hand to give to the child so that I could take the old dirty one away and wash it off while the child could use the new pacifier that had already been washed.
You don't always have to wash the pacifiers everyday but you really should especially if the child has the flu or cold or other sickness.
I washed the pacifiers using some hot water and a bit of dish soap and made sure to rinse off the soap fully before giving it to the child.
Some people only wash the pacifiers once per week which is okay as long as it doesn't fall on the floor.
If the pacifier falls on the floor then it needs to be washed right away before the child uses the pacifier again.
You can also dip the pacifier into some boiling water for a few seconds which will also kill off any germs that the pacifier may have on it.
If you notice the pacifier has any cracks on it or is coming apart then you should get a new pacifier for the baby or child because they can become a choking hazard if you give the child a pacifier that is coming apart.