I've never seen a disc with "sealed bearings" that were truely sealed. EVen with roller bearings, they generally aren't totally sealed. They are "vented if you will, to allow grease to push out dirt and moisture when you grease them. Dirt WILL enter the bearings if you don't grease often. All the disc manuals I've read will tell you to grease until you see it coming from bearing area. Deere offered some models of disc's with "triple-lip" seals at one point. (I'm sure other brands did also, but I'm familiar with those models of Deere products) Not sure what they sell now. Even those would let excess grease out after only a few hours in the field. Seals on disc bearing are intended to keep dirt out, but are generally "one way seals" that allow some grease to bleed out when in use.
"Boxings", or those grey iron bearings need to be flushed with grease often too. That excess of grease is what protects them from dirt entering the bearing.
For occasional use, a disc won't wear a seal OR bearing very fast. Under hard use, say in a full-time AG use, they wear rather quickly. One day in the dirt, and they aren't new anymore. Flush with grease. Shoot it in the bearings 'till you see it. Just use good judgement on when "enough is enough". On my current disc, an International Harvester model #350 (10') wheel disc, with roller bearings, I use 1 to 1-1/2 tubes per greasing. The excess around the bearings do attract and hold some dirt. The excess of grease also PROTECTS the seals from the dirt actually making contact with them. As was already mentioned, excess grease forced past the bearing area is what "purges" any dirt from the bearing.
Following that maint. program, my old IH #350 disc worked around 80 to 100 acres of crop land for over 20 years. Most of that ground required multiple discing's to get ready for planting. The original bearings went approx. 18 years. Set #2 is still OK.
