We just returned from CBR a couple of weeks ago. We found it was best to be at bus stop at least 45 minutes prior to the time you want to be at the parks, especially for MK since it seems to be the farthest away. We usually aimed to be at the bus stop an hour early, but sometimes arrived only 45 minutes early.
On our first day, before we knew how long the travel time and wait time was, we thought 45 minutes was plenty to get from CBR to the GF. It definitely wasn't, because you have to get to MK first and then get on monorail to GF. But for just getting to the parks, the 45 minute time frame worked for us.