Shouldn't take as long as the last one!
There will be a few more upcoming. It's the price of success. This one is for a software upgrade. This site has grown much faster than expected, with the result that we might exceed server capacity in a few days time. Therefore we will need to migrate to a (much) larger and (much) faster server soon, which will mean taking the site down for a while. The upside is that it means that we might be able to allow larger image uploads.
Also going faster than expected is software upgrades. These usually involve at least a restart to activate, sometimes more. We'll try as much as possible to consolidate these different operations into as few down times as possible.
The point is, please be on the look out for service interruption announcements.