How much time it will take you depends on several factors:
1. Do you have a pre-made deck? If so, is it a high quality or average deck?
2. Are you making your own cards?
3. If making your own, how many and how thorough do you plan to make them?
4. Have flash cards worked for you in the past? (Anki will inherently take much longer for someone who doesn't learn that well from flash cards versus someone who soaks up flash card info like a sponge)
Personally, I'm an anki fanatic, but that's because it's worth the time to make my own cards because it work for me. I know a lot of people that tried making their own cards just couldn't keep up with making cards and actually having time to study and review them, and others who Anki helped, but it wasn't great so it wasn't worth their time to make their own, but worth it to use someone else's cards.
The way I do it is I make cards as I go through a set of slides and/or syllabus for the first time. I basically consider it the same as making an outline but in flash card format. I'd say on average, it takes me a full 1-1.5 hours to convert one lecture to cards with an average of 60 cards per lecture, but I make extremely expansive, detailed decks; I make them comprehensive enough that you could learn the material without ever looking at the slides or syllabus they came from (and I only do that because I need the active attention/learning that flash cards force you to do vs. passively reading notes/slides). Obviously some lectures are easier than others. I spend the first half of my day making new cards and looking at new material for that day, and I spend the second half of my day doing old cards that I've already made. I review ~200-300 old cards a day, give or take, over the course of maybe 3-4 hours, but I have Anki repeat cards until I know them cold so a lot of times I see cards with concepts that aren't quite sticking multiple times per day over multiple days. As tests get closer and I don't have new decks to make, I can do anywhere from 500-900 cards in a day (depends on the subject and how diligent I was with my daily card reviews). But again, I thrive on the active learning and the constant changing of pace. Some of my friends think I'm insane for doing that many in one day, especially considering how thorough my cards are. But hey, if it works, don't try and fix it, haha.
Now, I'm a bit of an oddball because of how I basically convert lectures to anki and then hit cards over and over. That's why it takes me so long. If you on the other hand are only going to selectively make cards only for the things you can't remember, it will take you far less time to make them. And if you can get your hands on pre-made decks either from others or potentially online, that will take the least amount of time (assuming the cards are written well), but there's also no guarantee that things you need the cards for most will be incorporated in that deck. So there's no short answer really. Depending on what you want to get out of it and your resources, Anki can eat hours upon hours of your time, or take up only a small fraction of it.
