"Achilles" is medium/high variance so while 600 spins without a bonus is more than possible, it's typical of this type of slot.
RTG's "Real Series" slots percentage settings take one of 3 forms. Based on memory, these are 97%, 95% and 93% set globally across all slots at a casino.
BUT...bear in mind they all have progressive jackpots which will not only reduce the actual expected return in normal play, but will increase the variance - by how much on either I couldn't tell you. If you want regular bonus rounds, then go "low variance" with something like "Derby Dollars". And avoid "Rain Dance"
RTG controls the % settings, not the casino as some widely assume, and a casino has to request a change, justify it to
RTG, then wait for around 4-6 months for it to be applied - if it is considered justified (don't ask me how they decide!!).
Slots Variance
And now for a little informative spam: I put the closest approximations to variance for around 200 slots on
Slotjunkies - use the dropdowns to find slots close to certain variance types. (Sorry if you use Firefox but I didn't have the time or inclination to deal with it's petty layout quibbles
)