It depends on how you are getting the input.
If using the vba "Input()" function then it might be accomplished with a timer (Application.OnTime()) and the SendKeys function, but I've found the SendKeys to be tricky and unreliable.
Another way would be to create your own User Form, then you could set a timer and Close the form when time is up. Don't forget to kill the timer in the UserForm.Terminate event for when the user has entered some value before the timer event triggers.