The 5-man dungeons, on average, are supposed to take an hour. At present, based on the average gear level available, they should take at most half that time, even the new ones.
As for raids, a lot of time goes into assembling it, especially with a pick-up group (PUG) raid, but even guild raids people tend to not be there on time, need to update addons, etc. Also, remember that if the raid wipes, all that time is wasted.
In general, though, I'd assume 10-20 minutes per successful boss encounter, including trash.
A surprising number of raids have one real boss in them, like Eye of Eternity, Onyxia's Lair, and the Obsidian Sanctum (also Vault of Archavon, as everybody only wants the newest boss, really...). These (with the exception of EoE, which is a rather technical fight) are quite often pugged.
On the other hand, instances like Naxxaramas (16? bosses) and Ulduar (13 bosses) have many more and can take some time to clear. These are less likely to be completely pugged, but it happens.
Brad