As a general rule of thumb, I recommend practising 4 or 5 days a week:
|Beginners||15 mins a day|
|Grade 3+||30 mins a day|
|Grade 6+||1 hour a day|
Above this, practice time will increase depending on what you do with your instrument. For example, when I was preparing for grade 8, I used to practise for 2 1/2 hours a day. Expert performers, with large repertoires to built and maintain and who make a living from playing music, will need to spend much of their working day practising.
That said, it’s important to bear in mind that the quality of the practice, rather than the quantity, is what really makes a difference. For example, an hour rushing through a set of pieces, playing the easy bits quick and the tricky bits slow and full of mistakes will be of much less worth than twenty minutes working in detail on those tricky bits, ironing out mistakes and getting them into your muscle memory.