I remember 3.5e had it where the mount stayed 2 hours/paladin level.
Now, Pathfinder Paladin has it where you also summon the steed, even more than once per day, but how long does it last? What's the point of using it more than once if it can stay forever? Can it be dismissed as if it were extraplanar?
The rules on it seem very vague.
The paladin's ability to call his/her mount no longer has a durability - it's basically an "instant" effect. The mount appears wherever it is called, from wherever it was, and that's it. It just teleports, effectively.
This is useful for getting your mount somewhere awkward to reach (up a cliff, through a tight space), though be careful if you need to get it back down/out/whatever again. ^__^
It can't be dismissed, I don't think, as it isn't extraplanar and has nowhere to go. Even when it gains the Celestial template, it only becomes a magical beast.