When i first got my guitar i wanted to be john petrucci, shredding like a nutcase but have now given up. I just want to be able to play some agressive riffs and polyrhythm style stuff like some of the slower mmeshuggah songs. How long do you think i would have to practise a day before i could keep time, play strange timings and play some cool grooves? I can't really practise too long each day because i bloody hurt my arm a while ago.
dont give up dude just keep at it and before you know it you will be shredding like a nutcase and oh yeah cant you wait for your arm to heal before u get back to playing guitar
It really depends on how you will practice, because a good practice that takes an hour could be better then a bad one that takes 4 hours. Look on the internet for different plans then edit them to suit you. I've found if you find something that is effective and fits your playing style and goals, you can practice for as long as you want and improve everytime.
2 hours a day? but i think maybe an hour would do?
and odd timings aren't technicality its more getting the groove for those timings , you can practice those without a guitar
That all depends on your innate ability and also the quality of your practice time. If you focus hard on achieving those things, then hopefully it won't take too long, but nobody can give you an accurate estimate.

I don't think anyone should force themselves to practice longer than what they find enjoyable. It's bad for you. If you have to really push yourself to play for extended periods, then playing for extended periods was not made for you.

btw, you said one of your goals was 'keeping time'. Do you mean as in basic, being able to play simple 4/4 stuff accurately? If so, then you're no where near ready to attempt polyrhythms and suchlike.
well there is on time that you can put on practise more is always better. but i would say at least one and a half to two hours a day. and you can play like john if you work hard at it and dont give up. remember that playing the same song over and over wont make a great guitar player quick, work on several diffrent songs at the same time. practice one for 20 min then go to another for a while. it help keep it interesting so you dont get board and say **** it. it takes a lot of detication. its not easy but once it gets in your head its hard to get out.
just play when you feel like to play, don't force yourself.
btw watching some concert dvds by your favourite artists also helped me alot (youtube anyone?)

and remember: it is all about fun
just play when you feel like to play, don't force yourself.
btw watching some concert dvds by your favourite artists also helped me alot (youtube anyone?)

ya im with this guy on the dvd thing. helps get you motivated
Just practice as much as you can, whenever you've got spare time, as long as you feel like it though. if it's just a hobby you don't have to force yourself into it so much. btw you've probably 'matured' in your goals but still, i bet you would like to be able to shred like petrucci still? Don't completely give that up, maybe spend some time working on your sweeping and other shred techniques. Have you ever taken lessons? I've tried loads of different practice routines and I ended up using the same structure my teacher gives me, just longer than half an hour. Warm up, stretch, ease into it with sight reading, scales, scale licks, chord stuff, a song I'm working on. Usually the song is one that relates to what I'm learning. I can improve well if I do 10-15 min warm up/stretch, 10min sight reading, 45 min scales and scale licks, 30 mins chords, as much time as is left over for the song, and also some other techniques like sweeping and legato and vibrato. So about 2 hrs I usually end up playing. But, you're more interested in theory than playing like Petrucci so spend more time on the sight reading and related theory and less on the repetitive scales maybe. But, you can spend time doing mental/reading/written theory without a guitar as well, that won't hurt your wrist. Spread your practice out over the day, not just 2hrs at once. But as much time as you can afford, when you want to play, without any pain, is as much as you need to do. Different if you wanna be a studio musician or a teacher or a rockstar I guess but otherwise it's just like a sport, you don't do hours and hours every day of that do you? Although guitar's better :P:
I am not a good guitar player at the moment. When i mentioned about wanting to keep time, i meant that i wanted to be able to double things without having to adjsut and cut tracks for it to fit together. But i can keep good time if i have a drum track to play along to. I'm not really interested in playing solos at the moment, I just can't be bothered, so i would like to practise just rhythm guitar for now, it sounds lazy but a solid rhythm guitar player is very important to a band, solid rhythm guitars, drums and bass make the song.

Anyway, thanks for your posts dudes.
You should practice the time that you feel comfortable playing. Otherwise you'll be making no quality time practice at all. That's what I've been told and now I try to take it easy because I'm a beginner, so I try to keep getting better without rushing anything. To me, 2/3 hours a day is enough because I don't have many things to learn at the moment. I'm just getting myself into a better shape of playing, without bad habits or stuffe like that. Once I improve, I'll surely practice 1 or 2 more hours a day.