The Admissions Panel strongly recommends that you begin keeping records of how long you spend on each job, in hours and minutes. You will need this for your own purposes, so that you can calculate estimates when setting your own fees for a job, and for planning your work (such as deciding whether you have time to take on a project or when negotiating deadlines with a client).
You will also learn, if your records are detailed enough, how quickly or slowly you really do work on various kinds of jobs, so your estimates will become more accurate. HM Revenue and Customs also expects you to keep this information. It is used, for example, in claiming the appropriate level of business expenses for working from home (hours worked each month). As with all HMRC data, you are required to keep all your business expenses information for six complete tax years plus the current tax year. Further, should you ever need to claim benefits from the Department of Work and Pensions, you will find that some are only available to people who work more than a certain number of hours per week, so you will need to be able to show a history of your working time. You will also, of course, be well placed to complete your next upgrade application accurately.