In my experience this is always an issue when when works for the big accountancy firms (and probably the little ones too) - you are expected to work incredibly hard for peanuts on the offchance that if you make it through to Partner, you are made for life.
Your contract would say something about this. It'll probably say something like ' your nominal hours of work are x to z however on occasions you are expected to work the hours necessary to deliver your responsibilities. I would seek to negotiate your time in lieu now - before the evil date in January arrives.