There is no correct answer for this and there are many different opinions. Most think that somewhere between one and two sides is the best length. Sometimes the firm you're applying to will specify what they require but this is rare.
The best thing to do is make sure each point you make is concise and relevant to the firm you're applying to. It's ideal to get it down to one side if you have included everything. However, don't be worried if you have gone onto a second side - it's a really good thing if you have plenty to talk about.
When you've completed your CV, why not send it across to us for a complimentary CV check - email firstname.lastname@example.org and one of our membership services will give you some feedback.