You work your way up to full strength in increments of 1/4, 1/3, or 1/2 strength applications each week. It depends on the line-up & what strain you are dealing with. Some strains are heavy feeders & can handle a full on administration of nutrients whereas other strains are only able to handle 1/4 to 1/4 strength max during feedings all of the way through flowering.
If your plants begin to slow down in growth & seem stunted then that's a sign to bump up the feeding. If you see the leaves are yellowing around the edges then you have overfed them. Over time you will learn to do what is called "read your plants".