William,
It would be useful to know why you ask the question.
Is it to wax the car or detail it? i.e. Machine polish, followed by a last stage product, wax or sealant.
I agree with Mike that 2 weeks would be OK, but if detailing, 2 weeks is ok, but the longer left the better, especially if you need to remove moderate orange peel.
Cellulose, Synthetic and 2K Acrylic is readily available in the UK from several suppliers including this one:-
http://www.jawel.co.uk/No association, but I did purchase my 2K Acrylic from them.
They do include the following statement on their web page:-
"We still stock cellulose paint ideal for Industrial Use and restoring automotive cars. We are able to mix and match almost any colour for cars, RAL and British Standard Colours".
Download the free Health & Safety Data Sheets before purchase.