Get a plastic cup that you know how much it holds and pour it into a bottle marking the known values.
Although a measuring cup is pretty cheap.
using soil you should water as fast as the soil can take it and keep watering to get 10% of the water given as runoff through the drainage holes.
Using organic fertilizer you can't keep the solution over a day.
Using non-organics you usually can keep the leftovers but you would need to re-check your ph when you're going to use it.
In the pre-fertilized soils I grew in (that 4 brands I can remember right now, probably had more. all can't be found outside of Israel) the fertilizers provided was enough for 2.5-3 weeks in a proper grow. When I added guano powder to my soil before planting in the recommanded dose it lasted a week longer.
The rule of thumb as to rather add fertilizers or not is to start adding fertilizers when new growth is starting to become pale. Thats the most obvious sign of N deficiency. As a vegitating plant takes mostly N thats most likely the first deficiency to show.
If I was to grow again in soil (moving to coco, liking it much better) I would have used quite a bit of amendments such as bat guano (although it extremely rare around here), earthworm castings, compost etc. If you don't go crazy with them it can help you save a lot of expensive liquid fertilizer later on.
Although not related, don't forget to add a LOT of perlite to your mix. AT LEAST 20%. even if it comes with perlite.