Personally
I really don't see bumping as a major problem.
I think adding rules "could" cause a kerfuffle
I voted 7 days, if you are not going to change price/card. But I have no problem with shorter bump times if reducing prices and/or adding cards. However, that's were things could get "un-ruley." You'd get into how much a drop would be significant, etc.
I like (and try to keep) to the 3 posts on the first page rule. Simple to follow, for simple minds like me.