Go to Admin > Ticket > Close Ticket Workflow.

Here admin can set ticket workflow like Number of days,Send email to user and Ticket status.

When No of days is 0 that means the ticket will be closed immediately after getting status resolved. If No of days is set to 1 then the ticket will be auto closed after 24 hours (1 Day). The ticket status after which the ticket will be closed can be set from Ticket Status. Send Email to User will allow you to send email to clients about auto closing. If it is selected no then the client will not be notified  about auto closing of his ticket.