When the fruit appears. A truss is the bunch of fruit.

Yep, they mean when the first tiny fruits start forming.

Tomatoes are very hungry/thirsty plants. Feeding them young is vital for strong, healthy & productive plants. It develops a healthy root structure, stems & leaves that will be ready to support a bountiful yield once it's mature enough to bare fruit. They would be better able to fight off pathogens & blight as well. Feed your tomatoes weekly.

A truss is the branch from which the flowers & fruit develop. It's the 'bones' of the plant. Healthy bones = healthy fruit & YUMMY TOMATOES!

But, if you don't mind me butting in - just how much water do they need ? I fill the container below the pot every morning and wonder - if it's empty later - whether to refill it or not. ?
Or should one measure it out in litres ?

The bigger the tomato plant grows, the more thirsty it will be. If it has a load of developing fruit, it needs yet more water/food. It's good that you are very diligent about checking the soil. Just be sure it never dries out completely.

Thanks B.