Before your first session starts
Deciding how long to keep spinning depends on how often the Free Spins round arrives. That frequency is not known in advance, and no session can be used to predict it. The result of one session is random whatever the RTP is, so a long run of spins can end below the stake or above it.
Before you pick a stake, check the RTP in the game rules, because only those rules state which version this casino runs. A figure quoted on another site does not settle which version is running here.
Planning around the top prize means treating the maximum win as a ceiling, not a target. The ceiling is reached very rarely, and it is stated as a multiple of the bet, so the money figure rises or falls with the stake you choose.