Duration of NEW marker for content
The default is 30 days, per the NODE_NEW_LIMIT constant.

Nodes changed before this time are always marked as read.
Nodes changed after this time may be marked new, updated, or read, depending on their state for the current user. Defaults to 30 days ago.

Since use of that constant is hard-coded in node_mark(), I imagine you'll need to provide your own 'marking' system if you want to do things differently to the core version.