Personally, never found timing a problem. Granted I do sciences and maths at A level but I have always had plenty of time to answer the questions and even at GCSE with the essay based subjects I found I had plenty of time too. With A levels the exams are roughly a mark per minute and I know that works for a lot of people. However much time we gave to an exam there would always be people who felt it was not enough and would want more time. I know in a 10 hour CA at GCSE people felt that wasn't long enough to write an essay (2000 words). The time limit ensures that people should have enough time to think and write their answers but also mean that they have to keep time. They can't spend ages waffling or ages thinking. In reality you can't spend a long time thinking, you have to come to the correct answer quickly and correctly but you also have to make sure that if you are answering something you answer the question effectively. The example I always think of is that of a teacher. If a pupil comes up to a teacher and asks a question they don't understand, the teacher must answer it in a certain number of ways, instantly (or very quickly), correctly, and making it easily understandable. If the teacher takes too long to think it wont help the pupil, if the answer is wrong it really wont help the pupil and if the answer is confusing or contains a lot of waffle it wont be straight to the point and could confuse the pupil.
While I agree that exams are testing how much a student knows and should provide enough time to show that, students also should learn how to answer questions effectively so that they can do it in a set time. Life rarely gives you the opportunity to have as long as you want. If someone runs out of time in an exam they should think about why they ran out of time. Did they not understand the material so spent a long time thinking, did they not plan effectively or perhaps it was due to a lack of practice. For example in my physics class, we came out of a hard paper (1 hour long) but we ended up all doing quite well. One person came out of the exam saying that he felt the timing was a joke and there was not enough of it. In the mock papers we had done he took hours to complete a paper because he struggled with the content. Is that a fault of the timing of the paper? no, it was the fact he didn't understand and hadn't learnt the subject matter well enough.
Of course everyone is different and there are people who the things I have been talking about do not apply to and just do generally require more time to work and I think perhaps exams could be lengthened but not by much, 15 minutes or so maximum because otherwise it defeats the point of having a timed exam and equally you will get some students going, I finished 30-45 minutes before the exam finished and was sitting around for ages doing nothing, why couldn't the exams be shorter.
Personally after that long winded answer :P I think exams are good as they are.