> I realise this will depend on the party and levels.
You're right, it will. In fact it can vary a lot. There's probably a minimum time that it will take a well equipped party made up of players at the top recommended level for the instance... but you could shorten even that by going with a couple of well equipped level 60 friends ;)
To save time, don't worry about how long the instance will take. Instead, read the guides to see which quests you can complete in the instance of your choice, collect all (or as many as you can) of those quests beforehand, and go to the instance after reading the guides on how to best do the instance and how/where to complete each quest. That will be a much bigger time saver then worrying about how long the dungeon can take...
Two good places to start:
WoWWiki: http://www.wowwiki.com/Instances_by_Level
Thottbot: http://www.thottbot.com/
Hope this helps :)