How to answer 'To what extent....' questions e.g. 'To what extent can the period of Conservative dominance between 1951 and 1964 be viewed as the Thirteen Wasted Years?'

'To what extent' questions are usually followed by a statement or an assertion that expresses an opinion rather than a fact. You are being asked to test this statement against your historical knowledge in order to reach a judgement about whether you agree or disagree with the opinion. A good way to approach this is to compile a list of evidence supporting the statement and another list of evidence that contradicts the statement to see whether you are more in agreement/disagreement. It is ok to disagree with the statement, but the important thing is to use your historical knowledge to explain why you disagree. It is also not entirely agree with the statement, but again explaining why you think the opinion has strengths and weaknesses is the crucial point.
For example: 'To what extent can the period of Conservative dominance between 1951 and 1964 be viewed as the Thirteen Wasted Years?'. Firstly, I would acknowledge where this opinion was coming from and what it was referring to. In this case 'The Thirteen Wasted Years' is a Labour criticism of the Conservative's 13 year period in government. I would then compile a list of evidence that could be used to support this statement, looking into the economic policy, social policy and foreign policy of the governments of this period. In support of this statement I would emphasise how Britain missed an opportunity to be part of the European Economic Community and their preference for stop-start economic strategies over long-term economic planning. I would then compile a separate list of evidence that may contradict this statement, such as inflation and unemployment remaining low during the period. Through this process of weighing up the evidence in favour of and against the statement I can then decide 'to what extent' I agree, and select relevant evidence to support my judgement.