It’s a common question and the answer depends on you. Here is a story that may help you decide... I was exploring a new city looking for the art museum when I asked one of the locals how long it would take to walk there. He simply replied, “Walk”. I thought he had misunderstood so I asked again, “How LONG will it take to walk to the art museum?” He replied a little more firmly, “Walk!” I decided this conversation was going nowhere so I walked away with my usual long stride. After a few steps the man I had been talking to yelled “About 10 minutes”. I stopped abruptly turned around and said, “Why didn’t you tell me that in the first place?” He said, “I couldn’t tell you how long it would take until I saw you walk”. Until I observe someone practicing, I can’t tell them how long to practice. When someone practices piano efficiently, they require far less time than someone who practices mindlessly. If they play through a piece from start to finish a couple of times, then it will take a long time to learn it. If, however, they practice with intention, marking a little fingering and doing little 2-4 bar mini practices, they will learn a piece quite quickly. At university I used to learn the pieces I didn’t particularly like the fastest, so I could spend more time with the music I loved. Here’s how:
Play the music through the first time note perfectly. Don’t play a note until you are sure it is correct. If you need to, play it hands separately. This discipline gets easier when you see the results.
Mark any fingering that isn’t obvious. You may change it later but put down your best guess. Repeat the sections that you fingered, a few times a day. Those are the hardest parts.
Listen to a recording of the piece and hum along to get the melody and rhythmic structure in your bones and get ideas for your approach to this piece.
After (and only after) you have done the first three steps, start at the beginning of the piece and play it a little faster than you are comfortable but stopping (hopefully) before you make a mistake. Note that spot and move on.
You may have noticed a theme, avoid making mistakes at all costs. Every time you make a mistake you must unlearn it to correct it. I'm lazy and save a lot of time by not making them in the first place. You play what you practice, so practice mindfully and with intention and you will achieve the results you want in the shortest possible time. If you don’t know how to handle a section of the music Don’t Practice It until you have asked a teacher or mentor for help in how to approach it. Teacher’s love questions like this!
BTW, the answer to the title is somewhere between 5 mins and 4 hrs AND doing it everyday is key.