I can usually go a couple years without retensioning a new saddle, and then it needs tightening once or twice a year. Feel it as you turn it, and when it's tight, you're done. That's usually 1/2 to 3/4 turn per year, max.

If the saddle is really loose, you may find the bolt head rotates against the frame, and you don't actually tighten anything. If that happens, get a bit of sticky paint and dab on the bolt and frame. When it dries, you're ready to go.