The hard part (in terms of time) for me is creating the content. I cheat. I throw whole paragraphs and sentences in without clozes or refinement.
Then when I review them I think, "Well, that was informative but not answerable, what does 'easy' or 'hard' mean when I'm re-reading a paragraph?" So I spend some time refining them. This is iterative. The first pass on a paragraph of content might just be to split it out into several cards with one sentence each, nothing more. Then when I see the sentence cards (some time later) I turn them into clozes or Q&A cards. Then I realize they need images to really help so I add some. After 2-5 iterations they're useful and effective cards, but the work ends up being distributed over a more reasonable period of time (usually 1-3 weeks). Each refinement step only takes a minute or two per card, if that, especially as you get more comfortable with the editor.