It is a matter of prioritising. I often spend a week getting everything set up for a show (if I am notified about it a week earlier) however sometimes you only have a few hours available to do it.
Usually I do things in this order (and if I run out of time, it does not matter)
1. Work out rough production schedule (including "No Change" dates and submission dates etc)
2. Work out agendas for first production meeting and rehearsal
3. Create "Welcome to the show" packets (contains contact lists, health forms, HR forms, details about the show, details about the venue, history of the show and the period it is set in if needed yadda yadda yadda)
4. Make sure I have all my kit ready for day 1 (including lots and lots of pencils)
5. Create script extractions
6. Create rehearsal paperwork
7. Create prompt book
8. Create performance paperwork
The list is by no means complete, but gives an idea of the areas I work on. Some things like production schedule will only be rough, especially if you have not talked to the director, but I usually like to pencil in a no change date about 5 or so days before we move into tech, and I like to pencil in a "set design required" and "equipment requirements due" dates - usually the equipment lists about 2 weeks before tech week if possible, and set design about a month prior. After the first production meeting though, these things can change drastically. The outline of a production schedule should take no more than 20 minutes really.
The agenda for first meetings is usually fairly basic too - production meetings - welcomes, directors vision, each departments vision, budget, timeline etc. First rehearsal - welcome, your time, directors time, charactor discussion, design discussion, readthrough etc. Maybe an hour to put together.
"Welcome to the show" packets can go from the time it takes to photocopy a few documents for the entire cast, to upwards of 5 or so hours. One thing good about stage managing is you can reuse old documents.
Checking your kit should not take long if you already have one - if you want to put one together from stuff arround the house, maybe an hour or so.
Script extractions require you to have read the script once already (say 2 hours), then probably another 3-4 hours to do the extraction, so 6 hours.
Rehearsal paperwork - sign in sheets, call sheets, line note sheets, rehearsal reports etc - if you are making from scratch, 4 or 5 hours, if you are recycling, 10-20 minutes.
Prompt book - anywhere from an hour to 20.
Performance paperwork - sign in sheets, call sheets, performance reports, much like rehearsal paperwork.
It also takes a day or two to set up the rehearsal spaces and callboard.