chopficaro,
You can do whatever you want to, their your plants. But don't be surprised when your plants start developing
nutrient problems. Depending on the water volume and size of the plants, I would go anywhere from 1-4 weeks between changes. This article I wrote will help explain the relationship between plant size and water volume, factors that affect nutrient uptake, as well as some of the issue you'll encounter when the reservoir is to small for the plants, and/or you go to long between nutrient changes.
What size reservoir do I need
P.S.
You said you were planing to grow tomatoes. Tomato's don't have a veg and bloom stage. Their continuously fruiting, they do both stages at the same time. And the so called flush at he end, won't do anything for tomatoes.