Hello Pavlin
Your setup looks very nice!
How many watts is that fluorescent lamp?
In my opinion you will be fine with the fluorescent until the plants outgrow it. That is best not done by fixed schedule. Instead, allow the size of the plants be your guide.
I notice you are using large rockwool cubes ... be watchful of pH with those. they tend to raise pH unless they have been thoroughly soaked in a mildly acidic buffer.
For your question on how often to change the reservoir ... the Wilma systems do not seem to have a large capacity. i would change the nutrient solution once a week, no longer, until the plants are big and consuming the liquid quickly. monitoring pH and EC (ppm) will also give you a "feel" for the lifetime of your nutrient. it os like cooking ... the schedule is less important than the look of what's in the pot! Best luck to your grow. cn