It's tough to say. Are you powerleveling or since you say 600k p/h I'm assuming your going to be banking most of the drops? It could take awhile, I donno how long exactly. Someone else will have a better understanding. Powerleveling would be a lot faster, obviously. You just gotta decide on which method you're going for.
It personally took me a month to get 85 slayer from scratch without any economy. So yeah, as a845 said probably like 3 weeks depending how much resources you have.
Powerslaying with cannon and task skipping would take you 3 weeks minimum tbh. The cash/hr will have dropped by then as there are a lot of people going to be camping abbys.
Powerslaying is like what all the other guys said. You do the quickest method which is usually using a cannon and whatever other melee style and you don't pick up drops unless their a substantial amount of GP. Like for instance: you get ogres, rather than banking all the big bones, you just leave em alone, or you could bury em I guess since that's pretty quick.