Dennis @ PerfectForms
A form will time out if it has been open for the amount of time indicated in the time out field regardless of activity. When that time limit is reached the user will be given a warning with the opportunity to continue filling in the form or close without submitting. The warning stays open for 3 minutes before automatically closing the instance. The time out can be increased to 9,999 minutes (over 6 days). You can also give your users a “save without closing” button.
A time out is important because it prevents a user from perpetually locking a form instance.