how much do plat n dimonds play ?

On the contrary. At diamond there is really nothing to grind anymore (unless u want to be tryhard and go for >d1 50 lp /master/challenger) ppl play like 1-2 games per day at maximum. Even thoose who play on more regular basis tend to smurf rather than playing on high acc.

Personally i play 1 game every 3 weeks to stop decay and thats it.

I play one or twice a day lol