Before taking off a member of the crew will arm the exits. If opened, the emergency slide will inflate in seconds.
According to Traveller: "It is a requirement that an aircraft emergency slide must fully inflate in just six seconds."
For this reason, it's also a requirement for flight attendants to send a signal to airport staff or emergency crew outside of the plane, letting them know the door is armed (i.e by using red or orange tape).
As the publication points out, "Some aircraft types have other ways of indicating an armed door, but every aircraft has some method of warning those outside when a door is armed."