When DefaultTimeToLive is -1 then your Time to Live setting is On (No default) Time to Live on a container, if present and the value is set to "-1", it is equal to infinity, and items don't expire by default.
Time to Live on an item:
This Property is applicable only if DefaultTimeToLive is present and it is not set to null for the parent container.
If present, it overrides the DefaultTimeToLive value of the parent container.