Help! Yellow Leaves, Fertilizer Question

Hey, my two week old seedlings had yellow bottom leaves so I fed them last night. The leaves are getting yellower and one of them has a little brown spot on it now.

I don't know if I fed them enough since I used very little since I wanted to be careful about burning them.

So I want to know when the fertilizer should start taking effect so I know when I should add more or not. I hope it starts working soon or else those leaves are going to die. It was FF Grow Big at 1/2tsp per gallon, the recommended dosage for seedlings.

Just so you know, I'm not an impatient idiot who is like "OMG! Why didn't the fertilizer make my plants better overnight?" I just want to know how long it takes to take effect so I know if I gave them enough or if I need to give them more.

Thank you.

sound a lil like too much nutes, some pics would help though. What kind of soil you using?

I'm using FF Light warrior, it has no nutes. There is no way it was too much because it was like this before I fed them.

How long does it usually take for liquid fertilizer to start working? I don't want to give them more thinking its not working and burn them.

EDIT: just to clarify, I'm not asking why the leaves are yellow. I just need to know how long it takes fertilizer in soil to start working.