Depends on what type of grow medium you started with.

Too many variables involved to say "once", "twice" or whatever.

It all goes back to what was there to begin with, how much your plants took up, etc....even the size of your germination container will play a role in this equation.

How much water has your plant been given and how much water has run thru the bottom of the container......way too many factors to have a blanket policy.

Very, very individual issue.