It depends on the stove, a few quick jerks (5-6) of the shaker handle usually is sufficient. Once you see small glowing pieces falling out you should stop.
The bed size again depends on your stove. Most work well with about 4 inches. How often to add and shake depends on how you burn it. Most get in a routine and have the stove set so that it needs attention every 12 hours. You can add coal or shake it at any time.
If your new to it you'll get the hang of it shortly.