The quick answer to this question should be as much as possible, but like anything it's not that simple. Or should I say that complicated. Practice should be long enough to improve your playing and short enough to stop you from losing all enthusiasm for your instrument. It should be routine enough to become a habit, but progressive enough to become the most interesting part of your day.
The 10,000 Hour Rule
The 10,000 hour rule, made famous in Malcolm Gladwell's outlier book, states that an artist requires 10,000 hours of practice before he achieves professional and even world class standard. There are only 8,700 hours in a year! So even if you practice 3 hours a day religiously it would still take you 10- 15 years to master your instrument.
The word master is the operative word. You can still become successful before you reach anywhere near the 10,000 hour mark. Take a band like the Beatles. It wasn't until the end of their time together – they were band for around 12 years – that their contemporaries began to regard them as great musicians. Through regular performance, and the practice needed to perform to a good level, they slowly became masters at their respective instruments.
Does Practice Make Perfect?
Practice does not necessarily make you perfect at your instrument. If you practice all day, eight hours a day, you will just become bored and eventually lose all enthusiasm for playing. In fact experts suggest you practice only 2 – 4 hours a day. Even then you wouldn't get better unless your practice is both progressive and routine.
Taking this a step further, over the last few years there has been a lot of discussion about whether the 10,000 hour rules really works. Even if you practice for 10,000 hours, you still might not be very good.
Whilst the whole putting a number of hours on the amount of practice you actually need is very general, what is known, if you don't practice, you won't be very good. Practice as much as you can as there is no such thing as too much.
In these terms practice is not about working harder, but working deliberately. Set yourself goals and work out ways you can achieve them. The best goals are very specific and related to the level you are currently at.
For example, if you are a beginner guitar player your goal could be to learn all the major chords. If you are an intermediate guitar player your goal could be to the learn the chords and how to play your favourite song.
Once you've achieved your goal you can move on to the next stage. And the next stage should be an extension of what you've just learned. In that way you are continuously progressing.
Just as important as deliberate practice is deliberate rest. There is only so long anyone can maintain their concentration. Professional musicians for example may practice up to four hours a day, but they will only practice 60 or 90 minutes at a time.
In addition, anyone that really wants to become good will know the importance of sleep. It is very difficult to get into a routine and to concentrate on your playing if you are always tired. It may work for a while, but after a time the majority of people just will give in through sheer fatigue.