Speaking as a former British Sapper, I would say it would take no more than half a day. This allows time to unload the explosives, probably in small barrels, from their carts, tie the expolosives onto suitable points to demolish the bridge, lay the fuse and detonate it. This all assumes that the detachment were technically competent to place the right amount of explosive in the right place, particularly if it was an old stone bridge, which would need charges tied below the arch of the bridge, and may require constructing some form of platform to support them.

Normally bridge demolitions are prepared in advance, and then actually fired sometime later as covering force troops withdraw over the bridge.

Rod