The default timeout period is 10 minutes of inactivity. If you would like to disable the timeout:
From the main Registration Dashboard click on "Modify Event"
Scroll down until you see the "Ignore Registration Timeout Feature"
Click "Yes" and there will be no timeout feature on the registration pages.
If you would like to add a custom timeout period:
In the Modify Event area, there is a "Registration Inactivity Timeout" field. Here you can put the time in minutes that you would like to be the inactivity timeout period for registrants.