When to Transplant seedlings

When roots are visible at the side of the root ball you need to transplant it to a larger container.
When you wait to long there's a great chance your plants becomes "root bound".
A plant becomes root bound if it is allowed to grow in the same pot for a long time.
In nature, a plant’s root system naturally extends three times the length that it does into the sky.

Source : Transplanting seedlings