If it was an investigation...
then the FP would have been valid for a lifetime ... (they may soon move to other tech like dna/eye print or body chips which may last more than a lifetime)
but for BCIS, which still uses the "anghoota chaap jamana" manual, the ink was supposed to go faint and become illegible after 15 months. Now even though they use compooter to store the FP, they still think that the compooter image would fade and hence the re-FP.