i'm Looking for option to seed for a particular period of time only like 10-15 days. so is there any custom option to seed for only 10-15 days?

You could try Settings>Transfers>General>Stop seeding on upload minutes above and set it to 21600 (24 hours * 60 minutes per hour = 1440 minutes per day; 1440 minutes * 15 days = 21600 minutes).
Obviously, this won't stop the torrent after 15 days unless Tixati is running straight for 15 days.  If you are running Tixati only 8 hours a day - that's just 480 minutes to give you an idea. The upload will be stopped only after it has been running for 21600 minutes.