How much ink you add will depend on how empty the cartridge is. Unfortunately, unlike an ink cartridge or piston filler with this feature, there is no ink level window. You can certainly see from the length and diameter that the capacity is about 1 ml. If you are just topping up and need to use it right away, don't overfill it. Otherwise there is no problem with over saturating the sponge. The excess just flows out of the breather hole near the tip. Make sure that if you overfill you don't do this over your 18th century persian carpet. If you notice ink overflowing out of the breather hole wipe of the excess and let it sit upside down in an empty glass for a few hours or overnight. The sponge will absorb the excess ink, especially if it has been dry for awhile. You will note that a fully filled cartridge will last much longer than a new one purchased at the store.
Note the experience above with using ink from other manufacturers. Unless you remove the sponge and rinse out all of the ink there may be a reaction between inks of different manufacturers. Differences in pH, salt concentration, pigment type, or lubricants may cause the two inks to react and precipitate. No problem really, just throw it away and use a new cartridge. If you do this and find an incompatibility please note it here so others avoid the same incompatibility issues. I have never had a problem using MB Royal Blue as a fill up.