That really rather depends on the size of the venue, the equipment required and your selection of add-ons.  A small evening-only disco can be set up in around 30 minutes, whilst an illuminated dancefloor will take that amount of time on its own to assemble.  I normally try and find a point in the day where I would have around 90 minutes to setup.  That way I can test and soundcheck without getting in the way of staff and guests at your venue.