All depends on who does it, my neighbours, father and son, did theirs themselves, in between doing their own jobs, it took them about three months, but i'm sure a company could give you a fairly accurate time quote.
The team of lads I often work with on larger projects have done a number of loft conversions. 1 joiner 1 electrician 1 plasterer 1 labourer and myself (Plumbing/Heating). All mucking in together. Usually around four weeks work depending on how much steelwork and strengthning is required and how soon the inspector can get out to pass each phase/stage is a factor aswell. I might add though that we are not one company who work together everyday just all self employed and pass on work to each other or work together on big projects. So maybe a specialist team who work as a unit would be faster than us. So as a guidline only I would assume this is fairly accurate timescale.
I had hoped it wouldn't take much longer than a month to 6 weeks. We are planning it for sometime in the early New Year and will be using professionals (not attempting anything like this ourselves are we are incompitent!)