A friend of mine, who makes vandyke and salt prints and uses a facial tanning lamp like mine, says that salt prints do require longer exposures, indeed, but not overly so. His exposure times with salt prints are about twice the times of vandykes, which means at most twenty minutes.
So it appears that your tubes may not be the right ones for this process. You say you've used a tanning lamp before having built your own. How were your exposures then?