Really interested in the game. It looks amazing. I wanna know how much of my time I put into this game. Is there replayability? Or do you just finish the game in 1 playthrough and never touch it again? What is the avg. palytime of the game? Howlongtobeat.com doesnt have data yet.

It's similar in length to Hellblade: Senua's Sacrifice, I would say. Maybe a little shorter.
A completionist run is definitely going to take you a couple hours longer than the about 9-12 (very thorough) hours I put into Hellblade.

Finishing ECHO once unlocks Ultra Hard difficulty and an alternate mode called Legacy, iirc, which lengthens the Echos' memory by an additional cycle (2 as opposed to 1).

There's also other unlockables like alternate skins and whatnot, if you care for such things. Those are unlocked by the aforementioned completionist runs.

I honestly wouldn't hinge the decision of whether to buy ECHO or not on length and replayability. Its beautiful execution and the core mechanic of essentially playing against yourself should be unique enough to warrant a purchase, imho.
Me personally, I've just been waiting for something as completely different and more philosophical in nature as ECHO and I had not to think twice about picking it up the moment it popped up on gog. Thanks again to Ultra Ultra for bringing it here.