If time is not an issue then the answer to this question is simple, as often as possible and as long as possible. However if you have a particularly busy schedule you may be wondering what is the least amount of time you can practice your instrument and still get results.
I promise you will get a lot of different opinions on this subject depending on who you ask but I think most professionals would agree that frequency is the key here. For example, 15 minutes every day would be better than 2 hours twice a week. If you are a new player you are still developing dexterity in your hands which you need to fret and pick. Daily practice will make a big difference. In a perfect would 30 minutes per day should get you some amazing results.
Try to pick the same time each day so it becomes a routine. I practice every morning before I go to the gym, no one is awake then and I don’t get any disruptions. Another benefit to practicing early in the day for me is that my mind is rested and fresh so I learn better in the morning. In the end, you have to work with the time you have, so be as consistent as possible, and practice as long as possible even if that means only a few minutes per day.