Feeding milk replacer is the most expensive (if you're feeding a quality milk replacer - and if you don't, the calf won't fare well) and labor-intensive part of raising a bottle calf.
I'm with BS - I want 'em weaned off the bottle as soon as possible - once they're eating 1.5 lb of a good quality calf-starter ration per day, you can stop the bottle/bucket, cold-turkey, and work the grain ration up fairly quickly.
I've never fed one longer than 6 weeks; most were weaned shortly after 4.
Every time you pass by him, cram a handful of calf-starter in his mouth, and keep fresh feed in front of him at all times. A ration with enough molasses in it to make it especially palatable is helpful.
Studies on dairy heifer development(and it probably carries over to steers, as well) suggest that grain-based starter ration consumption during the first 2-3 months is much more effective in getting rumen function going than hay; most recommendations I've seen over the past 10 years or so are to withhold hay feeding from those calves 'til they're at least 2 months old.
http://www.uky.edu/Ag/AnimalSciences/dairy/extension/nut00031.pdf