Before the first spin of Gates of Hades
Deciding how many spins to play depends on a question no one can answer in advance: how often the bonus round comes. The frequency is not known before the first spin, and the result of one session is random, whatever the RTP figure says.
Deciding whether the RTP version on offer suits you starts with one detail: which version this lobby runs is shown only in the game rules. Read the game rules before the first spin, because the versions listed on this page do not say which one is active.
Counting on the top prize is a choice to make with open eyes: the maximum win of Gates of Hades is x10,000 of the bet, a ceiling reached very rarely. Each stake has its own ceiling in money, so the cash figure changes with the bet.