I was just editing my Photos Library, deleting a LOT of old Albums dating back as far as 2004 that had migrated through several versions/revisions of iPhotos, Photos, etc. and I happened to have iCloud open in a Safari window at the same time. I had been working for perhaps ten minutes when a popup appeared on the Photos icon in iCloud announcing "the database was undergoing maintenance and was not available".
iCloud icon.
That should give you and me both, some idea of how quickly the synch occurs. By the way, those changes appeared on my iPad (which was locked and plugged in) by the time I could get to it to check.
