iCloud Drive is a file hosting service for devices running iOS 8, OS X Yosemite, or Windows 7 or later.
When will we be able to start using iCloud Drive in WIndows PCs? I can't seem t find that info anywhere.
A guess would be Sept 17, date of iOS 8 release; though that is just a guess.
It may be with Yosemite, for which no date has yet been announced, afaik.
"When it's ready!" might be a better answer ;-)
Try this, I believe it answers your question directly;