When you experience something new, your emotions and abilities change after a few days. Use this expression to talk about these changes:

I was super slow when I first started. But by the third or fourth day, I had gotten the hang of it and was almost as fast as Melinda.

It's common to include "started to ___" with this expression:

By the second or third day, I started to get tired of eating Thai food and just wanted to have something familiar.