From what I've learned playing basketball for years, it isn't really about how long you practice, but how you practice. Sounds cliche, but hear me out.
I know a lot of players who will practice free throws and three point shots for hours, but they're mindlessly doing it. Repetition helps to an extent, but you really have to focus in on why you're missing/making it. How are you hands positioned? How hard are you throwing it? Are you getting good rotation? Are you drawing power from your legs? Are your elbows kept in?
Same with cards and magic. You can practice a sleight for hours, but constantly doing it wrong actually just reinforces bad habits. You really need to focus in on what is working and what isn't. Performing a sleight properly 8 out of 10 times is good I guess, but it's much more powerful and long lasting if you can perform it 10 out of 10 times AND know exactly why it's working every time.
Josh said he can practice while watching television, but the INTENSE practice is when rehearsal time comes. This makes sense. Practicing and performing a sleight unconsciously sounds good because that means you can do it without looking. But your brain works differently when you have to perform and are actually conscious about the moves. It'll be harder to perform in front of an audience if you've been practicing unconsciously (like in front of the tv) because your brain doesn't have any information to recall when you perform it live in front of people. Practice only matters for as long as you can stay focused on it. For most people that is anywhere between 10-60 minutes. You'll have to figure out how long that is for you.