depends on the strain and size of the plants, but like every couple of days expecialy in bud. The reason the leaves are yellowing and dieing is because the plant is sucking all the nutes out of the leaves to try and produce flowers.
Maybe next time try using a weight gaining additive along with your base nutes in the flowering cycle. I use big bud by advanced nutrients. All so some carbohydrates and your plants will love you.
it definitely doesn't look very good. just a few pointers for next time try using sunshine mix #4 or promix and a good hydroponic nutrient like advanced nutrients.