I have 19,000 photos. I purchased extra storage and set up the synching for optimize mac storage. It has been trying to upload....12 hours later it is a tiny fraction completed...I intend to delete 1/3-1/2. tech support had recommended uploading first...?
Re: how long does it take to upload a very large iPhoto library to the iCloud storage?
Would depend on what your internet connection is. Generally, the upload speed given by most ISP's is quite low, esp when compared to what the download is. 19,000 photos is a lot, though the more important info is how much data is it? Based on some rough guesses and assumptions (which I generally avoid), I would say that it will take at least several days, if not longer (or much longer). If you can tell us how much data it is you are uploading, and what your upload speed is, someone may be able to offer a more accurate timeline.
Best advice is to just wait it out, check it every 12 or so hours, just to make sure the bar is moving, even if slightly, option 2 is to stop it, and delete the ones you wanted to delete, then start it again. Though, even 8,000 photos is quite a few, and will still take quite a wile to upload.
Not sure why if you intended to delete a lot support told you to upload first. Every picture you delete is one less that needs to be uploaded.