Optimal viewing for structural observation is daylight. As a functional bridge, time of day doesn't impact enjoyment, only traffic volume.
For structural observation, daytime offers the best visibility for discerning the engineering details of the Royal Lelystad Bridge. Early morning or late afternoon might provide interesting light for photography, if one were so inclined.
However, as the bridge is a functional piece of infrastructure, there is no 'best time' in terms of visitor experience or special events. Traffic volumes will be higher during peak commuting hours (typically 07:00-09:00 and 16:00-18:00 on weekdays), which might impact the quietness of any observation point, but generally does not affect the bridge's accessibility or operation.