I think you will need to follow the practice as learned from the experience from other boiler mfrs that have used P91 for the last 2 decades- some rules of practice work in their shop environment, and some don't. If you guess wrong, then you will have a costly repair bill to address when final hydro occurs and cracks galore ( younger brother of pu$$y galore, who starred in an early James Bond film) shows up.
First of all, for thick welds, there is not permitted to be any cooling below 170F prior to PWHT, so you must be addressing thinner welds.
The 2 issues that I am aware of that affect this time delay are SCC stress corrosion cracking and plain old shop stresses. I know of P91 pipes that have been left out in the Florida rain for months prior to PWHT, and no issues arose. And there are other cases where P91 bundles stored indoors had SCC , likely related to condensation whose droplets included concentrations of sulphuric compounds. As far as shop stresses are concerned , any loads placed on these welds ( and just pushing the bundle around will stress some welds) while they are in the extremely brittle state prior to PWHT can lead to cracks. It is my understanding that the initial use of higher Ni filler was related to adressing this last issue, as it improved the low temp ductility of the weld prior to PWHT, to reduce shop re-work.
