better off with a specialist - they have it down to a fine art - I can recommend someone if you want - his typical turnaround is approx 1 week start to finish.
I've used Enda Mooney - A&E Building Services (black vans with gold lettering) - 087 929 0259. He specialises mainly in Attics but also general building. As for the 1 week turnaround - that's why I would recommend a specialist - he puts all materials (steel girders/stairs etc) on pre-order so his teams arrive, do the work and leave - no waiting around. Obviously different attics will require different effort/time but he'll be able to let you know how long.