Hey,
I have Foxfarm Grow Big nutes for veg, what I want to know is when should I start using nutes and when using nutes how does it make your plant better?
The "when" depends a lot on what your soil has in it to start with. Most commercial soils have at least one month, sometime 3-4 months, worth of nutrition already in them.
So, certainly no sooner than a month. If your soil is pre-fertilized, I would suggest not adding anything unless you see an actual deficiency crop up. Putting nutes on top of a pre-fertilzed soil is a recipe for disaster. This site is loaded with "Help!! My plant has curling leaves / brown tips / spots...etc." posts that come from this very thing.
Be aware that Foxfarm and most other brands are very acidic. You need to rebalance the pH back to around 6.5 before you pour it in. A hassle and another argument for not using them unless really needed.
If you decide to add anything, be sure to start no stronger than 1/4 strength of the recommendation. Wait at least a week to see how it "takes". And be careful of the "add nutes every other watering" bromide. That comes from old-school growers who use 5-gallon buckets as pots and water heavily once a WEEK. So "every other watering" means every TWO WEEKS. Fertilize more than that, and you're on a road to ruin.
Others will jump in and say I added X, Y, Z and G at double-strength or whatever and got great results, so your full of shit, Kriegs!!! Just be aware that a plant can perform in SPITE of us as well as because of us, and nutrients+results does not mean the nutrients caused the result.