For how long can you use the Siaar pads? When I add surface I do 4 sides... So I was wondering when will be the best time to buy new Siaar pads. I always do wet sanding, spraying the ball with water and also the pads.

My estimates (Siaair vs. Abralon) are about the same as Xrayjay's.
A 4000 grit Abralon pad lasts 3 - 5 uses, depending on whether you're doing a full thorough refreshing or a brief light pressure adjustment. Siaair lasts about 25% longer,  roughly 4 - 7 uses. The lower grits (rougher surface) pads last longer the lower the grit.

The easiest to tell is the 4000 grit. When they are "used up" (the grit level breaks down to around 5000+ grit), they will put a shine on the ball, where a new one will still make the ball look dull, even though it is very fine - 4000 grit.

All abrasives wear, that is, break down into finer, higher grit numbers, with EVERY use. Only time and a sharp eye, as to the appearance or the use of the ball will let you know when the break down is too severe.
Hint: put a mark on the back of the pad with a black permanent Sharpie with each use. It will help you keep track.

thanks for your help guys! And I asked precisely because I was using a 4000 Siaair pad and the ball looked shinier than usual so it made me think about its life time. I ordered some more and i'll be starting the sharpie system lol

Thanks! 👍