You'll never know, and need to expect the unexpected.Probably 30 seconds or so for the band.
When you add in the parade as a whole, I'd guess at a good few minutes, but this is a good example of where the fact I'm only in any position of authority or responsibility for the band (rather than the whole parade) is a problem - I have no idea how many people or vehicles will be behind us!
Estimating it based off our remembrance day parade (which doesn't go over any level crossings!) and it would be over 5 mins to clear a section of road the same size as this crossing.
Whether this summer gala has more people or fewer than remembrance day is something I just don't know at this stage.
In addition to the band there will be your 'supporters', folk with strollers, wheelchairs, cameraphones, distractions.
A ship has one Captain, and everyone knows their place.
I feel for you. Stay safe, boringly so if need be.