How do you know when parsley is ready to be harvested.?

Answer 1

Parsley can be harvested over and over again from the same plant if you don't take too much at one time.
A good rule of thumb, is to leave 3 leaves for every one you pick. If your plant only has 4 fully developed leaves you can take one.
After the plant reaches a fuller maturity
you can take almost half the plant with out killing it. Because it's an herb, you should be able to harvest off of it for several months and give them a break in the winter months.

Answer 2

When it looks like it will taste good on your dishes..