Possibly too late to help you, but for the record: I asked PayPal tech support this question, as I couldn't find it in the docs either. They wrote:
Each transaction is associated with a token that expires after 3 hours if the buyer didn't complete the checkout. This isn't usually modifiable but can be extended by contacting a Business service representative ( we don't recommend to extend it for security reasons however. the token can't be less than 3H in any case)
So it's a MINIMUM of three hours.