Little and often
Ideally you would aim to practice daily. Exactly how much depends on what you're trying to achieve but a baseline is about 30 to 40 minutes a day for beginners. However this can be split up into 15 or 20 minute segments and often this 'little and often' approach can yield greater dividends. This works particularly well in the early stages where you're developing things like hand position and basic motor control. Such elements of technique need to be second nature and doing them frequently and consistently really helps achieve this. These short sessions also guard against errors brought on by loss of focus and fatigue. Another advantage of this approach is that it fits in with the pressures of modern life and can be scheduled around other activities competing for your attention. As you progress as a player you'll find more intensive practice is required but by that time you'll be capable of longer sessions.
Goals and achievements
You'll be given clear and concise goals during your lessons and these will be tailored to you personally. Some goals will be short term whilst other goals and areas of study will be more long term. The trick is to enjoy the process and not fixate on one particular end goal too much.
There is no end goal - only the journey.
It's not uncommon for people to start playing the guitar in order to play one specific style or tune only to find that once they're capable of doing so they've shifted priorities or tastes away from it. That's all good though and just part of the process! The guitar is such a versatile instrument you'll be exposed to a wealth of new music. This only adds to your journey of discovery.
Learning an instrument is a complex and not always linear experience so it's better not to have narrow expectations. As long as you're enjoying the process you're winning. Zen Guitar is an excellent book on the subject. It really puts things in perspective and shows that it's the same for all of us! The fact that you never stop learning is what enables musicians to derive endless satisfaction from the process.
Having said all of this some guitarists will want to take grade exams. This might be purely in order to gain entry to music college or university or simply to quantify they're progress. Either way we can prepare and enter you for all the major exam board grades. For those looking to become full time musicians I have tutored many professional guitarists in preparation for college auditions.
Essential practice tools
A Metronome - an invaluable tool for developing your timing - an absolute must have. Here's an online one - COOL! Everyone has a smartphone and there's loads of good, free apps that will do the job. Wonderful!
A practice diary - There's many ways to achieve this either on paper or digitally. I have a private blog which I use to save ideas. I post quick videos of whatever I'm working on so I don't forget them and can come back to them at a later date.
A Electronic Tuner, pitch pipes or tuning fork - good in you don't want to snap strings or annoy those of us with perfect pitch. Again there are numerous good smartphone apps for this!
Picks, Plectrums or Plectra ! - For beginners I recommend the blue 1mm Jim Dunlop Tortex. Very forgiving! Beware though, make sure you buy a straight one. This material has memory and if manufactured wonky it remains wonky. Not good. Picks should be dead flat and have no rough edges. Along with string gauge and brand this is something you'll innevitably want to experiment with.
If you're an absolute beginner it's a good idea to do a little background reading just to speed the whole process up a little. Get hold of a copy of the Associated Board's Guide to Music Theory, Part I. This explains the very basics of music theory (note names, rhythms etc) in a very clear and easy to understand way and covers music theory up until Grade V. The easiest place to pick these up is online direct from ABRSM themselves or of course Amazon.