I had this issue too - very annoying!! Worst part was they kept coming up even if my iCloud was not full (had >300MB free). Is this just a marketing attempt at getting people to buy more storage? I hope note. Settings to adjust thresholds (timeliness and space remaining) would be nice.
Are you on Windows 10? If so, in the search box type in Notifications and select the "Notifications & actions setting" item. In the settings window, scroll down to "Show notifications from these apps" and find iCloud - then turn the slider to the off position. Notifications gone!!!