It depends, after a big burst of adding flashcards I'll have a big hump to get through for like 2 or 3 weeks. That might be hundreds of flashcards a day, that can take 15-20 minutes.

Right now I haven't added many cards in a while so it's more like 30-50 cards a day. Usually not even 5 minutes.

I wouldn't recommend doing hundreds of cards a day, especially if you're just getting into it. I'm just nuts.