time it takes to rehab a property

I'm looking at a property that needs a COMPLETE rehab, and to top it off it's in a historical district which makes it extra hard. on average how long does it take to flip a property like this. I mean new shingles,plumbing,electric landscaping, drywall. I was estimating alittle over a month. I have been told it would take 3 months or more by a realestate agent. I was wondering what everone else time frame is. I think the key is to find a contractor that knows your goals and that can meet them.