Nandrolone decanoate seems to have a variable half-life; the less in your system, the longer it takes to clear. This means it sticks around forever (can be detected up to a year after use).
Since nandrolone is also a progestin and can be difficult to recover from, I've come to the conclusion that NPP is a much safer way to go. Keep the cycle short (6 weeks or less) get on quick, get off quick, continue test for a couple weeks after the NPP to let prolactin levels reduce prior to PCT.
That's my $0.02 on using nandrolone safely. Same advice goes for trenbolone, stick with acetate.