I try to grease all my balljoints and tie rods at least once a month. I want the grease to ooze out since I can see old grease out and new comes in.
Also, that's why it says "Inwards" (I believe) on the rubber boot when you are installing because grease is suppose to comes out on the inside so it doesn't fall onto the braking area (I've read somewhere) where it poses as a safety hazard.
All the boot does is keep the outside elements away from the actual inside ball joint where the grease is suppose to be to keep it stiff. Keeping the rubber boot filled shouldn't be the concern.
But, the rubber boot is suppose to last longer than 5K miles.