To charge for an entire project or hourly I think is often debated among designers and developers. It is advised you track your hours, but at times this can be difficult for some.
Freelancer, contractor or just for personal interest Toggl is a great solution available for Free with Paid Upgrades available. Toggl is an application that works in real time and times your entries while you work on a project. What’s great about this application is you can divide work, work in teams, track your own pace, set billable rates and share time reports. The ability to have compiled time reports can help you see on average how long it takes to complete a project and provides more detail for invoicing.
Have you used Toggl? If so let me know what you thought!
For more visit : https://toggl.com/