Stock bearings have 20-30% grease
fill. This works great for bearings that continuously rotate and keeps drag down.
For more bearing life (but more drag) you can go to 50% fill. Flick out one seal, fill that side and replace the seal.
For slow moving or highly loaded bearings (headset and suspension pivots) you can do both sides and get about 75% fill.
More fill does mean more drag and more mess (grease bleeds out the seals). But it does make bearings last. Thicker grease protects better but drags more.
For maximum speed and minimum bearing life, see what skaters used to do. Flick out the seals, blow out the grease and run a few drops of oil instead. Not recommended for MTB!