You can set the Lead Threshold for each salesperson in the administrator. The Lead Threshold is the number of days from the last activity on the lead before it gets a Fire Icon next to it. Changing anything about the Lead Window and saving it will update the last activity. If an appointment is scheduled for a later date, the lead will not get a fire icon until the lead threshold past the event date is reached. By default, every salesperson starts at a 14 day Lead threshold.
Example of Fire Icon on Leads (with Activity Date greater than the threshold given for a salesperson).