Labs seem incredibly important for an inquiry-based classroom.
Some teachers stop doing them because of management issues. Three days per lab is one way to plan - day one for preparation and exploration, day 2 for the actual data collection, day 3 for analysis and communication. One lab a week seems to be plenty in terms of getting through the manipulation of the experiment. I would shoose labs that were not difficult and could be completed in the scheduled time. If you do decide to take on a more complex lab, I would suggest getting some assistance from a a teachers aide or volunteer.