It's an interesting question. I believe WAFC SigWx charts are a snapshot. Here's what Annex 3 App 2 says:
1.3.1 Forecasts of significant weather phenomena
prepared by WAFCs shall be issued four times a day for fixed
valid times of 0000, 0600, 1200 and 1800 UTC.
It doesn't rule out the idea of a validity window, but I believe there is no requirement, nor intent, to provide one. I can't find anything relevant in the SADIS User Guide, except similar wording to the above.