Your plants will tell you. Start at half strength, watch for signs of tip-burn.
Gradually work up to full strength, or if you are comfortable 'reading' the plants, keep increasing the concentration until you DO see the tiniest bit of tipburn, and note that amount. Then, give a heavy feeding with plain water. Refer to the amount that burned the plants, subtract ~10%, and that is your full strength nute that YOUR plants in YOUR system like.

I would feed twice a week and alternate with waterings of plain tap water (unless your tap water is evil and nasty) if the plants need more juice in between. Every week or so, try to give one watering that flows OUT the drain holes. This will help prevent soil salting.