While it varies from airline to airline, the likely scenario would be that airline employees or an airport contractor would send staff onboard to retrieve the items in the overhead bins and seatback pockets once they have been given the OK to enter. They would tag everything they find based on bin number or seat number, so as to have an idea of who is belongs to. Then take it somewhere for distribution to the awaiting passengers.
How long the passengers have to wait depends on the issue that caused the evacuation and how it takes to determine it is safe to unload the baggage.
Credit:stackoverflow.com‘