Bottle feeding is the most expensive and labor-intensive part of raising one. I always tried to get my steer calves weaned by 4 weeks, but would usually feed heifers out to at least 6; most dairy producers are now feeding heifers out to 8 weeks or so.
My rule of thumb was that once they were eating about 1.5 lbs of calf starter per day, I quit feeding a bottle and rapidly bumped their grain ration up to 5 lb/day as quick as they could handle it.
Having a good quality PALATABLE calf-starter ration is essential - gotta smell good, taste good to get 'em to eat it well. Every time I fed, or even passed by the pen, I'd cram a handful in their mouth. Usually changed it out and put 'fresh' feed in, daily; if they didn't eat it, I'd scoop it out and feed what they'd refused to an older calf.