What is longest distance without attaching a top plate of interior wall
In my opinion this should be done 3 times per span at a minimum.  So really it depends what you are building with.  If you use your stock 2x4s that are 8 feet then 3.  But If you get longer 2x4s I think you can also go with 3, given there are no breaks.  

(This is also assuming that you can fasten the bottom every couple feet)