based on the various storage threads, they should be fairly stable for a long time. unfortunately for any sort of verification of long term storage you're going to have to wait a while.. remember nbome's aren't that old and as such not many people have had them in solution for very long at all.
Just follow the basic rules of keeping something stable and hope for the best.
If you've got 20mg of nbome and need an accurate way to get 1mg or less and the only way to do it is a volumetric solution well then you're just going to have to bite the bullet and put it in solution and hope for the best no? You can of course use lesser material but that only opens up a larger error potential unless of course you've got a true lab grade balance.