How detailed does it need to be: Down to the minute? hour? 1/2 day?
This answer will have to come from your own organization and it is highly dependent on how valuable the timesheet data is. Most organizations will find that a resolution of whole hours is enough. Others will want to get down to a half-hour resolution and some will want quarter hours. In very rare cases, time down to the minute will have to be managed. In TimeControl, you can create validation rules that ensure that data is not entered in more detail than is required.
If you're the person trying to answer this question for your organization, then start with less detail. The best practice we've seen is that if you start with more summary data (hourly for example) and then see if you've got enough detail to make the business decisions you want, then you don't burden the staff with collecting data at a level of detail that provides little value.