We wean as early as 4 weeks in the warmer months, and not usually later then 6 weeks at any time, as was said before, you can't just take them off cold turkey, you need to see that they are eating grain well. and if in doubt, you can always suppliment the grain with some dry powder milk replacer for a while. that will also help sweeten the grain up, thus they should eat more.
Of course if you get a stubborn one that just wont eat grain as well as he should at first, you may need to continue the bottle for a bit, but give him periods of time were he will feel hungry and would be more likly to eat grain. You may want to wean down to a single bottle a day, preferably at nite, so that they are quiet through the nite, or you can start mixing in greater amounts of water until that is almost all they are getting, that in itself will produce the hunger as well.
Just don't starve any thinking that they will all wean off at the same time and do as great as the best ones you have. there is no cookie cutter operation when it comes to bottle feeding calves.
We suppliment all of our calves with inexpensive yogurt from day 1 (walmart plain or vanilla)...it's almost the same as probios, and helps to not only kick start the rumen, but it very nuitritional as well.
Remember, that what ever feed changes you make with your calves, and cattle for that matter needs to be slow, meaning that you want to give the rumen time to change it's 'flora' to be able to properly digest what you are feeding it.