How long does a liquid have to gone until the uv rays not be able to see it

Question

you know those special flash light looking thing that csi uses

to see liquid that has long been wiped dry how long does a liquid or water need to be gone until the liquid will be undetectable by the uv rays

Answer 1

This is dependent on the substance in question.  For the most part though, the answer is not until it has been thoroughly cleaned.  Blood, s***n, and other things with DNA are very difficult to completely remove without destroying whatever they are on.  Also, water won't show up under UV light unless it is contaminated.  Some cases have had detectable DNA samples after 30 years of sitting in an evidence locker ;-)