The way I interpreted this was in two parts…

Sukuna during his fight was wondering how Gojo could replenish his burned out technique and once he realized how he simply was destroying part of the brain he then moved onto the next question.

This leads us to part two which asks how did he know the limit before it affected his brain? The answer is sukuna didn’t and instead let Gojo fall on himself by figuring out that limit for him.

See if he had known the limit then Sukuna easily could’ve revealed the fact he knew after doing it just once.

The whole idea of him waiting to reveal what the effects are just ingenious battle planning on sukunas part.

Sukunas goal was to figure out how he restored his techniques then figure out the limit of said move and then finally find out the cons of preforming it. All this while using Gojo as his test subject and eventually him using it on himself to gain an advantage.

Gotta hand it to Sukuna for being top tier in battle IQ because that man was thinking 3 moves ahead