Is the re-entry permit office at Suvarnabhumi Airport open 24 hours and how long does it take to obtain it?

If you are not in Bangkok then try and get your reentry permit at your local immigration office. Happily got mine at Kanchanaburi Immigration.

At BKK use International Departures 2 and it is found after security, down the escalator before the Immigration desks

Besides what Tod said it also happens that there is only one officer and several applicants

Yes the re-entry permit counter is open at Suvarnabhumi Airport (& Don Mueang Airport too)

Now as far as how long it takes? That answer is; it takes as long as it takes :o There's no telling how many people are going to be ahead of you or if you'll be the only one walking up to the counter. I would say it usually doesn't take more than 20 minutes.

Remember they have two price points,

If you take the filled out tm8 form, a passport photo, copies of your visa, extension, previous entry stamp it's only 1,000 for a single entry

OR

You can just pay 1200 (a 200 baht surcharge) and give them your passport with nothing else