I only grinded twice to he heart stopper bonus spins, so it is kind of hard to say. For me 1st time took roughly 1200-1300 spins, 3 bonuses triggered and few half decent base game wins on the way. Maybe 250 euro loss (1 euro bet size) before the heart stopper bonus which paid out 169x. So not good end result, but not disastrous either.
2nd time I went in with little smaller bet and good that I did, because it was pretty rough. 2 crap bonuses that didnt pay out during the way and no big base game wins. I don't know the spin amount, but over 300x loss (0,60 bet size) before heart stopper and it paid out 15x. So that wasn't so good
All in all it is so volatile game anything can happen. You could win big base game or get a sick normal bonus and reach heart stopper spins with ease. On the other hand you can lose 100x times bet very quickly and without collected hearts. Very big sample size would be needed to give you a better answer. However I think 400x bet size starting balance gives you a fairly good chance to get the 40 hearts needed. If you run unlucky you might fail, but I think in most cases you should reach them.