A buyer's agent may schedule a few different appointments with the same buyer in a given day. Many times they know the approximate time they will be at your home but by setting up a time range like 8:30 - 9:30, the appointment can occur in, the agent is able to let their buyer take their time in the homes and account for traffic and delays. 

While they may only be in the home for a few minutes, the range gives them the flexibility to take their time and view what your home has to offer. The window of time can be shorter or longer for different appointments. You can always work with your agent to set up minimum and maximum appointment lengths. 

 Buyer's agents that use the ShowingTime mobile app are able to let you know when they have left your home.