Hi -The simple answer for the use you're choosing is that you do not need the oils in your bloodstream - that you simply need them to connect to the olfactory sense receptors causing the allergy.
Yes, in general, there's no need to overdo it. If you can smell the oil strongly enough, you're getting enough in you and don't need to keep 'hitting the bottle'
Another option is a 5% dilution of Blue Tansy massaged into the sinus areas (if you don't mind a little blue warpaint) ...which would absorb in 10 min. or so. This is the most frequently mentioned remedy for hay fever.