[Support request] A question of timing

I asked a question before but I’m not sure I explained my self correctly.
So it’s day 11 of the grow ( time to turn on my auto pots, I have 2 ) so I make a mix of nutrients – they have a mixed life of 1 week according to the instructions. I’ll make 4 litres as a guess. By day 17 my nutrients needs to be replaced. I have no way of gauging if the 4 litres was enough/too much or too little. How do I go through my first grow knowing what’s enough before my nutrients becomes unusable. I chose this system because I don’t have constant access to my grow.