According to Anki, I'm averaging 10 minutes a day on my language vocabulary deck. It does happen in a series of short, 1-2 minute bursts when I'm sitting on the toilet or whatever.
To that end, another useful thing I've discovered: Reviewing flashcards (and, spaced repetition) is not a good way to learn. It's a good way to reinforce your memory of things you've already learned. The real learning happens when you make the flashcards. The time spent focused on each concept so that you can decide how to break it into a series of smaller factoids that are right-sized for a flashcard, and coming up with whatever mnemonic devices you'll be using to help you remember this concept, is where the learning happens, because that whole process involves a fair amount of turning it over in your head.
Meaning you really shouldn't ever use a pre-made deck. It may not seem like it at first, but it really is normally more efficient to make your own from scratch.