We have 3 different kinds of ink in our collection, and all of them are document proof in varying degrees. First of all, you have to be aware that the particles inside the ink which are responsible for creating the long-lasting effect can clog up the ink feeder (which is below the nib) or the ink tank, if the ink dries out.

Scribtol

The most resistant ink in our collection is the "Scribtol", our item no. 221 135.

Due to the above mention reasons (color particles, i.e. soot), this ink should not be used inside fountain pens. Instead, it should be used with drawing nibs only (i.e. those that don’t have feeding systems and are only dipped into the ink). This ink is opaque and very light resistant, i.e. a document proof ink. To write, you drip the ink onto the drawing nib.

Fount India®

A compromise is the "Fount India", our item no. 221 143. This ink is opaque and light resistant, i.e. it is also a document proof ink, with almost the same quality as the Scribtol because it also contains soot (but in lower quantity than the Scribtol ink). This ink can be used with piston fountain pens, as long as you make sure that the ink NEVER dries out. You have to regularly clean your fountain pen with cold water if you use this ink.

4001 blue-black

The least complicated ink which is still relatively light resistant, is the ink "4001 blue-black", our item no. 310 607. This ink contains iron gall, which makes it much more resistant than for example the ink shades 4001 royal blue or 4001 brilliant black, but due to the addition of special ingredients, you can still use this ink without qualm in piston or cartridge fountain pens. Within time, the ink will change its tone from blue to gray, but it will remain visible. It is not quite as light resistant as the Fount India. Due to the small concentration of iron gall, this ink will not damage your paper (which sometimes happened with historic ink made of iron gall.)

It is difficult to say how long these inks will last because it depends on a variety of criteria, for example how much ink flowed while writing (was it a slim line or a broad line), which kind of paper you used (how much ink did it soak up), and how many hours it was exposed to light. That’s why we can’t give a general answer to this question.

You can find more information in the Internet, for example on this page:
http://en.wikipedia.org/wiki/Iron_gall_ink