SIGWX charts are produced well in advance and are thus to be considered
snapshots of an ongoing simulation/forecast, based on the data available prior to production of the chart.
The chart, therefore, has no valid period per se, it is to be considered valid only at e.g. 06Z.
If you want to cover your behind, you need charts covering the whole period of your flight, so a 07Z-17Z sling would require the 06, 12 and 18Z charts to be carried.
Hope this helps
