I wanted to say that the simple answer is.... But there is not a simple answer for a number of reasons.

When you run WCG you get credit for each core that you have running and those cores in turn get through a different amount of work depending the speed and type of processor so this results in me, for example, getting 12 years in 2 years.

When you run wcg and complete the work, for some projects at least, you are one of a small number of people doing the same work. This is done to ensure that the results produce good science with no errors. For you to receive credit for the work you do the other people doing that same work must also complete the wu's then they can be compared and proven good. You may, therefore, receive credit some days after you handed in the work. Conclusion: any time of day is good and even if you run 24/7 you will see a fluctuation in the credit awarded each day.