I wouldn’t create an artificial time limit within which the shift must take place. The idea is to keep the show moving forward, and not disrupt the flow any more than necessary, but fast doesn’t always accomplish that.
Ideally, the director and design team will have some kind of concept for what they do or don’t want to see happening onstage during the shift, so it can be integrated into the rest of the show. Sound design can be particularly helpful; there have been plenty of times when we have sound to underscore a shift, and try to get the crew action to fit with the length of that sound Q. That helps the crew keep the same pace each night and the audience stays engaged. I’ve often had directors get involved in “choreographing” the scene change so that it looks good to them, either by telling me what they’d like to see, or working directly with the non-union crew/ASMs.
In your situation, I wouldn’t tell the crew about the length of the scene shift at all. I would worry that would make them nervous, and lead to people rushing about and checking their watches. As you find out exactly what needs to move in each shift, and how many bodies you’ll have to make it happen, start drafting your plan for organizing the shifts. When your crew arrives, give them Run Sheets detailing what they need to do, and keep them focused on completing their individual moves efficiently and consistently. Then you can watch them do the shift, concentrating on the big picture, and shave off seconds with specific instructions to each crew members on how they can do their jobs better (such as, “carry on the vase when you go to strike the chair, instead of making two trips”). I’d think that strategy would be more effective than a general “lets make this happen faster everyone!”