When you put your home on the market you'll be dealing with all types of buyers and real estate agents.  Most agents try to notify you as closely as possible of the time range in which they expect to reach your home, but sometimes that schedule is hard to maintain.  Here are some of the scenarios you might encounter with scheduled showings of your home:

The No-Show: 

There will be those frustrating times when a showing appointment has been made but they just don't show up.  There are several reasons this can happen, among them the buyer saw the area and didn't like it, the buyer never showed up or canceled his appointment to meet the agent, or they drove up and didn't like your home.  I always try to call the homeowner and let him know as soon as I do and that's the right thing to do, but it doesn't always happen that way.

The No-Time Showing:

There could be times when the prospective buyer is driving by with his agent and says "I want to see that house now".  These are often the best showings, assuming the price is in their range, but they can be inconvenient for the owner.  If the house is in top shape and the intrusion is OK with you, then go ahead and let them in with their agent.  If not, tell them to wait for as long as you think you need to prepare the house and then come back or schedule an appointment for another day.

The No Agent No-Appointment Set Showing:

If anyone ever knocks on your door without an agent never let them in!  Have them either call your listing agent or their own agent if they have one to come back in the normal way.  Too many bad things can happen when strangers enter your home without supervision.

The Late Show:

When an agent sets an appointment "between 1:00 and 2:00" this usually means they will make every effort to show up in that range but sometimes they have no control over what happens in homes they are visiting prior to yours.  I've had people stay a long time in another home, have issues with their children, need to stop for lunch at odd times or become ill during the day.  Don't let a showing agent set too broad a time window, they should have a better idea than that of their schedule and they can always call you again if they start to fall behind.  If you come home and they are still in your home, drive around the block before going inside so you don't make the potential buyers feel uncomfortable or rushed.