Duration of NEW marker for content What is the duration of the 'NEW' tag that is being added when creating new content? How do I change it so it would last until the content is edited, after that the NEW marker would disappear.

You can find NEW marker in red in admin/content

I do not understand how this works, content added yesterday and today are all marked as NEW which is misleading. I'd like to be able to change it.
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.