Noob grow

Different for everyone. At 4 weeks mine are close to going into flower. All depends how much room you have for the plants to stretch.

I see you grow in soil with BioBizz, so you don't have to do run off every time to avoid salt buildup. I usually water a bit lighter in the beginning, at that size I would probably give a liter every 2-3 days (when top 5cm of soil is dry). Once they're larger/ in flower, I simply water to slight run off, so usually around 10% of soil volume - for example for 15L pot 1,5L. I also grow in soil with BioBizz so you can have a look at some of my diaries for more precise reference, some have dates/amounts in the weekly notes.

I usually do it twice during a grow. Once just before flipping the plants, this is mostly to improve airflow in my tent because I trim lower branches as well. I then do a second round about 3 weeks into flowering, this time to ensure the growing tips where the heads are forming are exposed to light. As for your question, its stressful for the plants, so you should only do it when there is a reason for it, the only benefit to doing in the veg stage is to improve the airflow.