I use Lightroom on my MacBook pro and my library is saved inside Dropbox. I can easily share folders with a Dropbox link and my photos are synced everywhere. The Dropbox upgrade gives me a terabyte of storage and can be used for more than just photos.
Not sure. I have a very fast connection and my MacBook is always on. I wasn't paying attention but I think everything was uploaded in a couple of days or so.