I'd say you'll probably have a higher combat lvl than that by the time you get 85 slayer, cos i'm 101 with 83 hp, and slayer is the same rate as hp i think.
As for how long it takes, i guess it's partly how you plan your time. I heard the fastest way for slayer is the edgeville master and use cannonballs wherever possible on tricky assignments.
Stock up on glory ammies so you can return to him quickly without visiting heroes guild too often maybe, and get lots of food ready, unless you're buying it as you go. But what do I know, i'm only 57 slayer

good luck,
big goals are fine if you've got the commitment for them, helps you keep your focus and not waste your time doing pointless things on the game.
Edit: forgot to mention that some of my friends who do a lot of slayer said that a recomended purchase when you can use it is guthan armour, since you can do most tasks in one go, with no food trips.