I've noticed that some Descent Quests have one encounter while others have two. Are they equivalent in difficulty? How about time to play?

The Heirs of Blood quest "Rellagar's Rest" has two encounters. Each map is fairly small. The "Siege of Skytower" quest from the same campaign is just one encounter, but has an enormous map.

When planning to complete one quest per session, do experienced groups factor in less time for single-encounter quests? Do they expect an easier ride?

Addendum - the H&M quests seem to all be single encounter - and don't have large maps. While a large map doesn't guarantee a longer game, are H&M quests shorter in duration to larger book quests?

From my experience, apart from intros, one encounter quest is shorter than two encounter, but longer than each of those two encounters.

Agreed, The H&M quests are really more like two encounter quests, but the encounters are split between acts.

So: big book two-encounter quest > big book one-encounter quest > H&M one-encounter quest?

Do the small book quests (Labyrinth of Ruin has some two-encounter quests, Manor of Ravens seems all one-encounter quests) align with the big book quests, or are they more similar to H&M?

Cheers.

small box expansion quests can be kind of lengthy- they are very similar to big box 1 encounter quests.

As a side note it's usually in the overlord's best interest to choose one encounter missions over 2 because that's half as many search cards that the heroes could potentially get in most cases.