I put it this way-Undergrease and you change out the bearing in no time. Over grease in the wrong place and you push out seals. Undergreasing is probably worse if the bearing has any kind of hours on it. But we usually grease ours fairly often to make sure. Them rotor bearings are tough being as big as they are. Number of pumps is really hard to say, but I suppose the other poster probably has some experience here...
Using a hand power grease gun is a great idea for this bearing. If any resistance is felt, stop. But since it rests in a partial spherical metal hub, the grease can, if the fit is not new and real tight, be pushed out between the hub and bearing. Also, if the anti-wrap system is worn and you have had the bearing wrap up in the past, the inner seal may be compromised somewhat. So IMO, if you get 3-5 pumps in and it comes up against resistance, stop there and perhaps make it an annual zerk. But if you get much more than 5, check where the grease is going by dropping the door under the feeder and check for grease around the bearing with a flashlight looking up into the anti-wrap window. IMO. If you see grease there, then perhaps make it a 20 hour zerk IMO.
On my 1680, the bearing to hub fit is quite good and the bearing is only 500 hours old. I get about 3-4 pumps into it at the start of each season and that's about it.
One thing that I have found out is that you should only use the ih extreme pressure grease for this zerk and the one on the moveable sheave on the rotor drive . An old ih service man had tuned me into this years ago -- something about the additives in the grease not going gummy after a while . And for the rotor bearing I just give it three pumps once a week .