Hi,
I guess all shops will have their own timescales to work to. I know alot of companies advise 3 months for hiring and that is a pretty good rule of thumb. However it will depend on exactly what suits you are looking for. Some small independent shops may need more notice for hiring popular products. I have some hire ranges in my shop which are already fully booked for June and I have bookings up to December So I would advise looking for suits about 6 months in advance.
In terms of buying suits, it depends on the method you choose. To have a suit made it could take up to 3 months again to complete. There is no real point in booking them any earlier than that as there will be various fittings to ensure a correct fit. If you choose to buy a suit ready-to-wear, it is always worth trying to buy in the sales.
Marc