Before the first spin, not after the tenth
The 6x5 grid holds 30 cells, and the bonus round can arrive on any spin without warning. How often it comes is not known in advance. The result of one session is random, whatever the RTP of the version you play. The RTP describes an average over a very large number of spins, not the outcome of your next hour.
Two RTP versions of the game exist. Which of them runs on Spirit Casino is shown only in the game rules, so read those before you start. Each version has its own return figure, and the money values sit in the table.
15,000 times the bet is the most this game pays, and that figure is a ceiling, not a target. Reaching it is very rare, so no session should be planned around it. Every win in a round depends on chance, from the first spin of the base game to the last free spin.