The slickness of cornhole boards is a matter of personal preference, but there is a set of standards that many people follow to keep them fairly consistent from board to board.
Board Slickness Test
Board slickness is personal preference, but as a general rule of thumb, you should be able to place a bag on the playing surface (about 3/4 of the way towards the top ) and it should STAY when the back of the board is raised to a height of 24 inches (2 feet). The bag should slide off when the back of the board is raised to 30 inches (2 1/2 feet).
So, lay a bag on your boards and start your tilt. If the bags slides between 24" and 30" mark, you've done your boards right. If they slide too soon, you're too slick. If they don't slide by 30", you're too slow. ~rucraz2