well, in oregon, there are several dual sport motorcycle events I have gone to where the staging area was a sno park pullout on mt. hood. lots of us camped over the weekend every year. But that may have been covered by the permit.
Otherwise I'm going to guess the rules may be similar to a rest area, with the max stay of whatever oregon allows, I think it is either 8 or 12 hours.
In the summer you don't need to worry about a sno park permit. that I do know.
My son works for ODF, I'll ask if he has any info.