I would be cautious about using it in an essay, not because it's informal but because it may be seen as vague language. The reader may wonder how long is a 'great deal of time', so it might be better to be more precise in your wording. The first sentence below is better than the second sentence: 1. 'The parliamentary debate lasted almost six months.' 2. 'The parliamentary debate lasted a great deal of time.'

Hi Angela, yes can be used in both formal and informal settings, including an essay. This expression is more formal, than informal.