Everyone who is serious about their guitar playing should practise daily. What you need to work on is different for each player and will depend a lot on your level of playing.
Let me break down the obvious playing levels with a guide on the kind of things you should work on daily:
It is important that you enjoy any time you spend with the guitar. The difficulty in the beginning is to play material which sounds good and is useful to play. Any beginner should learn basic chords as chords will come back all the time and it is important to memorise the shape and fingering of each chord. Easy chords for beginners are G and Em, but apart from those you should learn all the open chords in the first position as well.
In the beginning you will have difficulty to get a clean sound out of the chord. Try to strum the chord in various ways, play a sequence of about three chords and focus on clarity of the sound and try to get a smooth transition from one chord to the next one.
Apart from chords play also single note riffs and short melodies. Most tutorbooks are full of these ideas. Play them and concentrate on the right fingering. Stick to the suggested fingering as your fingers will memorise their place on the fretboard.
Make sure you understand how the musical alphabeth works in relation to your strings, and memorise some of the notenames on the fretboard of the guitar.
If you are working with a teacher you will make a start with playing together with someone else. If you work on your own, look out for a friend who plays guitar as well and see if you can play together some simple songs. Making music together is the biggest part of making music and it will provide you with a lot of joy and satisfaction.
Players of this level should work on songs. Make sure you have all the right chords of any song you want to play and memorise these songs. Working from memory once you do know a song. Play from memory will make you listen more to the sound of your own playing and you will also get into the song, you will become one with the music and your guitar.
Work with tutorbooks which teach you about particular styles, look into different ways to accompany songs as this will make you a better, allround guitar player.
Do some sight reading from time to time. The aim is to read and not to memorise with this task. Once your reading will get better your sightreading will improve. Sightreading is a great tool and you will learn a lot from it such as songstructures, various rhythms and keysignatures.
Do some jamming with yourself, just mess around with chords and riffs. This part of your practise is mainly about fun and using all your skills at the same time. Save this kind of playing for the end of your practise session. If you start with jamming you may not want to progress to any other forms of playing. Try to stick to a certain amount of time dedicted to work and jamming.
Who is experienced eh? Let me put it in this way: If you struggle with any basic musictheory related to your own guitarplaying you still need to do some work. If you play and you often have difficulty with certain chords or songparts or whatever, you need to work on your technique.
Any experienced player can move up to becoming an excellent player but you need to be honest and iron out all your weaknesses in your playing. It will take time but it is also a enjoyable experience to see how your playing will develop over time.
Any musical growth will not happend over night. All of your favourite guitarplayers have worked hard, and most of them still to to this point today.
All the suggestions I mentioned above for beginners and intermediate guitar players also applies to experienced players but you can apply it to a higher level.
If you have never looked at any sightreading, get yourself some tutorbooks today and start. You will enjoy what you will discover as you will enter a new world of guitarplaying.
If Rock guitar is your main style of playing, try some bluegrass fingerpicking and some Classical playing. You can do these styles on any electric guitar, just set up your amp for an almost acoustic sound and have a go. Most guitarplayers today do have more than one guitar so my guess is most people who read this will have access to either a nylon-or steelstring acoustic guitar.
You can try some Slideguitar and working with different tunings, all of this will take time to get used to but it is worth your while if you are serious about your guitarplaying.
My list with suggestions only serves as an example, you know who you are and what your level is. The point of this article is: Do not play the same old things you have been playing for a long time, work on various things at the same time as it will keep your mind fresh and your fingers alert.
The overal goal is to entertain yourself with your playing and if your keep a healty balance of variety of material your playing will improve over time.
Good luck, stick to it and hope to see you soon again.