I don't think there's any way you could possibly come up with an 'average span of time' for an obsession.
Some are mild, some are severe.
The obsession will last until you, your brain, your body realize and understand that it is not logical to be obsessing over said obsession.
thank u.. so its based on brain de-sensitization.. right.. when the brain says its had enough then obsessions go away.. i like also in the replies thus far..that obessions are mild and severe never really look at that way... i always would recall my 1st remembered case of ocd.. and that to me lasted like 3 yrs .. the firse couple yrs was severe to moderate then 3 or 4th yr if it did continue then was mild and very mild... so i assume thats the going rate of OCD in most ppl .. i guess i am guessing..lol