I personally attend theater just to watch scene changes. There's nothing I hate more than dipping to black while watching a tech crew bump into each other for a minute and half while the band vamps.

Solution? When I directed Treasure Island, almost all the scene changes happened in full light while the action continued downstage or around the moving set. Our longest transition was going from the town into the ship. We choreographed it to music and I actually had to slow the cast down so it would happen in time. Applause at the end. Another transition from the island to the ship had pirates fighting on the moving pieces.

The one change that happened behind a closed curtain went from an empty stage to the inn which was 30' wide. It happened in 8 lines from the actor in front of the curtain. Audience gasps at the open.

These should not be an afterthought handled in hell week. They should be choreographed just like your dance numbers or blocking. You wouldn't let an actor stand onstage for a full minute in silence because it would kill the momentum of the show. Same for set changes.

Reading some of the scene changes posts did remind me of when we did "Noises Off".  Of course those scenes changes were longer than 30 seconds, hehe.  But the audience loved watching the run crew do the change during the intermissions.  We left a decent level of light on the set (for the crew to see) but also because the audience loved seeing those huge set peices turn and roll across the stage.  Several times the end of the scene shifts got applause and by the end of the run they had it down to less than 5 minutes from what started out taking about 8-9 minutes of a 10 minute intermssion.

Scene changes in SILENCE should be 15 seconds or less if at all possible.
Scene changes with MUSIC can be 15 to 30 seconds if need be.

If the amount of time required to pass IN THE PLAY between the ending scene and the following scene is short, then the scene change should be as short as you can manage.
If the amount of time required to pass IN THE PLAY between the ending scene and the following scene is LONG, then the scene change may be a bit longer, just to reinforce the passage of time IN THE PLAY.

Creative scene changes can be entertaining.  EXAMPLE:  In Neil Simon's "Plaza Suite" we had only 1 intermission between Act 1 and 2, so we had the stage crew come on dressed as hotel staff (bellhops, waiters, chambermaids) and perform the scene change between Act 2 and 3 under full lights with music playing over.  It took more than 15 to 30 seconds, but provided a bit of visual diversion so it didn't seem long at all.