Ah man the goal is to get to where you can run the whole time without a res change (as my friend does) or at least all of veg in one res and all of bloom in another. I go at least 4 weeks and try to do just 2 res changes in my grow. Dumping weekly makes it more difficult to know what is actually happening but if you run sterile you don't really have to understand how hydro works and can dump weekly and get great results.
When you get good and Hydro and understand how it works all you have to do is top off. My plants will go through more than a gallon of high concentration nutes a day when in heavy veg growth or transition and early bloom. So if you don't top of it gets critically low in a few days. That causes massive problems unless the res is supper huge. That said if plants are not running fast then you wont see this.
The nute solution is based on the stage of growth. In very early stages you want different nutes then when it is in strong Veg which is different then in transition which is different than bud formation / stacking which again is different then resin top off.
So if you go look at good fertilizer websites, like Advanced Nutrients, they have feed charts that explain how these things change for those stages. Now they say "use this product in weeks so and so and now use that product". So then you need to do some digging to figure out what that means.
In general you need no nutrients until there are a few sets of fan leaves.
Then when veg really starts you want to start low with a solution that has a high ratio of nitrogen. (The concentration you should use is based on the level of photosynthesis which is based on the number of leaves and light intensity so until there are a lot we don't want a lot of nutes in there) All the nutes are important and there are 3 main Macro and ones a ton of Micro nutes needed. You need them all through out all stages of growth but the ratio of these change.
Then during transition you start lowering the ratio of nitrogen to a very basic level.
As bloom takes off the Phos. and Potassium need to be run up.
And this is why they sell different nutrient solution bottles for different stages of growth.
Near the end you want to stop feeding nutes and have it use up all the stored up stuff in the leaves. Below is a chart that shows you a lot of data on the stages of growth but notice the EC line especially. Also the pH is not static throughout the grow. That is to aid in the proper nute uptake at the particular stage. Some nutrient companies do a good job of adding buffers to hold the pH stable for you so they have to over dose and under dose some things to get the ratios right at that pH.
