I get this question from just about every beginning student. That’s good! It means that people are putting serious thought and planning into practice.
Drawing made from a performance of the J.S. Bach Prelude from Suite No. 1 for Solo Violoncello
I recommend that beginners practice no more than 15 minutes at time. That will go by like nothing! I can hardly eat breakfast in 15 minutes. 
I do this because we really want to help create good habits. Playing a string instrument uses muscles in ways that we don’t normally in day-to-day life. This will help to prevent injury and promote healthy playing habits. Just like beginning a new exercise routine, you need to start off easy and work your way up. Even as you get to the point of playing longer pieces and practicing more each day, most people recommend taking a 10-15 minute break each hour of practice.
So you your time, body, and mind wisely!
Image from Flickr by gshowman used under a Creative Commons License.
Ten Nothern
Amen.
All instruments are a very physical affair, and there’s definitely “weight lifting” that goes on.
Also, I find the 15 minute “flash” practice is handy for people who are really busy, especially if you can do 15 minute sessions multiple times per day.
kareensyl
It is so true! When I first started, I couldn’t even practice for more than 10 minutes! My bow arm hurt and the fingers on my left hand were in pain. But then as I persisted, everything became easier and now I can practice for half an hour without feeling much pain
kareensyl
Btw, I need to buy a set of new strings for my cello, do you have any recommendation of certain brands?
blochhead
Good, I’m glad the pain has stopped! That is always a good thing…
I’m glad you asked that about strings- it’s been a while since I’ve had new ones put on, I need to do that myself. I had to check because I wasn’t sure but what I have now are Jargar Blues on A & D and Pirastros on G & C. I think it’s the Pirastro Permanents. I would love to try the Larsen Wire Core for G & C, but they are extremely expensive. D’Addario are supposed to have some nice strings too.
Thomas
Jargar strings are very good.
Jonathan
I try to practice a min of 30 mins..but its usually and hour..but more like every other day. I have a hard time finding time at the end of the day..when I’m not completely tired. I have to steal the time away from my wife because when she is home when I’m practcing she wants me to do things..and I have to say noooo..this is my practice time.