Diazepam can blunt psilocybin effects for many people, though responses vary. Some notice little change, many report fewer visuals and less emotional depth, and a smaller group feels strongly muted effects.
How long this dampening lasts can extend beyond the calming period. Because diazepam and its active metabolite linger, reductions can persist for days. After occasional low use, some notice less dampening within several days. After regular use, reports range from roughly 1 to 3 weeks or more. I cannot verify a precise timeframe for any individual.
Skipping a dose for 24 to 48 hours may not remove the effect because of the long half life. Some experience partial return of intensity, while others still notice clear reduction. It is unpredictable.
If you are considering any change to prescribed medication, discuss it with your clinician. For a practical overview that summarizes timing considerations, see this Q&A: diazepam and tapering before psilocybin. Hope this helps.