I was comfortable scribing after 1 month (although still slow) and pretty good after 2-3 months. After 6 months, I was rarely stressed during a shift. Keep an ongoing digital notepad on your laptop to document everything you cannot keep up with, and you can stay a little later than the doctor during those initial shifts to perfect your charts.
Even if you dislike your doctor, NEVER be afraid to ask for repeated information, a physical exam finding you were unsure about, etc. At the end of the day, if that particular patient sues your doctor, he will be thankful in court that you made your chart as accurate to your ability as possible. Your "annoying" questions will remarkably increase the chart's accuracy, which may become extremely crucial later on. Also, physicians typically read over their charts in-depth after each shift, so your doctor will eventually notice the effort you put into each chart and (hopefully) begin to respect you more. In my particular ER, the doctors were awesome, but some took longer than others to warm-up to new scribes. It's a little frustrating at first for some of them to go from a veteran scribe to someone new, especially the doctors who aren't as computer-friendly and rely more heavily on their charts being completed for them.