On average (assuming you only have sanity grinding in hand and the other 59 cards in your deck) you'll hit around 6-7 chroma on a cast of sanity grinding with that deck. You have quite a bit of variance, however, from 0 to 13.
Or, to put it another way, you could play 2 sanity grinding's and get around 6 for 3 if you're lucky, or you could play curse of the bloody tome for 3 and get 2 per turn until removed (likely a min of 2, as it's unlikely they'll have enchantment kill open pre-upkeep) or dream twist for 1 with a flashback of 2 (net 3 cost, net mill 6 cards, but far more flexible in dividing up the mana than sanity grinding). Sanity grinding does have a higher instantaneous effect than most 3 cost mill cards but instantly milling 7 isn't going to end the game, repeated milling effects (like archivist and crab) are far better because they allow you to keep pressure up even if you don't draw mill cards.
Everything's in spoilers because its discussing the preview cards for the next set. Out of respect for those who want to avoid spoilers everything about unreleased sets is kept in spoiler tags.