if i created a portal with a mage, then logged off (as if i want to use this portal by my other char), will the portal disappear once the mage logs off? or it will stay for its duration?

It will either disappear, or you will be unable to use it. There are two types of portals: one for the mage's personal use which disappear with the mage, and one for a group. You need to be a part of that group in order to use the portal too. I've tried many a times to take a left over portal. Never works :P

It will disappear. Ask around in a city in trade channel for a mage to make a portal for your other character.

the portal will disappear