Ditto what others have said:
Cyanotypes will last as long as the paper they are made on.
The "New Cyanotype" process is much better than the traditional method. See http://www.mikeware.demon.co.uk/cyano.html
Tannic acid toning seems to be very robust. To test it I stapled a toned cyanotype to the deck railing out back. Cleveland doesn't get much sun, it is true, but after a year it looked as good as new - paper was a bit wrinkled but still strong and of a piece.
The toning only affects the image and doesn't color the paper. However there will still be minute amounts of cyanotype pigment in the 'white' portions of the image and these will turn color with toning. When the pigment is still ultra-pale blue it just 'brightens' the paper - see fabric bluing as another use for prussian blue cyanotype pigment http://en.wikipedia.org/wiki/Bluing_%28fabric%29
I use Bienfang Graphics 360 for alt. process. It is a brilliant white, smooth, but rather thin - sort of like tracing paper. 11x14 is about as large as one would want to make what with having to handle the stuff wet. It drymounts very well. And it is cheap - $10 or so buys 50 sheets of 9x12. Cyanotypes can, and should, be very cheap to make. I have heard the paper has changed recently, I don't have any experience with the new stuff; I should probably buy a pad just to see if it is still suitable for alt process.
Gallic acid toning doesn't form the same compound as iron-gall ink. Iron-gall ink was known to the ancient Greeks and parchment and papyrus documents written with iron-gall ink have survived 2,000+ years. The ink has a bad reputation as a faulty recipe was used for making it in the late 1800's in the USA - with atmospheric moisture it would eat through the paper - the insides of handwritten o's, a's, d's, b's ... fall out of the paper, leaving a hole behind. If you wash the print well after toning to remove excess acid there will be no problem. Washing wasn't something one did to handwritten documents in the 1800's. See http://www.realscience.breckschool.o...onGallInk.html and http://en.wikipedia.org/wiki/Iron_gall_ink