I figured this needs to be it's own topic as this is coming up in every Surplus XP thread. The following is a description of the Surplus system as it stands through Beta 3. Hopefully open beta will either turn this system into a reward instead of a punishment or do away with it altogether.
The 8 hours (15 hours) of XP is just a relative term. A lot of people seem to be confused by the wording they used in the official announcement. You don't necessarily have 15 hours to gain XP.
What they did was determine how much XP THEY thought you could earn in an hour of play time. For simplicities sake, let's say that amount of XP is 1000XP. Then they decided the average CASUAL player will play no more than 8 hours a week and thus earning no more than 8000XP per week (1000XP per Hour * 8Hours per Week). A casual player may never see this XP cap depending on what they decide to do with their time in-game. However, players that are intent on leveling may see this cap in the first couple hours.
Now they decided to allow the hard core players a little more XP by allowing players that exceed the XP cap to continue leveling, but only for a little while. For every threshold you reach past the XP cap you will gain Surplus that will continue to grow until you gain 0 XP.
This applies to both your Physical Level XP gains and your Class Rank XP gains. If you reach the 0 XP cap for your physical level you will not be gaining any XP for your physical level for the rest of the week. However, if you reach the 0 XP cap for your class rank, you can simply change your class and continue gaining class XP (for the class you switched to), but you will still NOT be gaining XP for your physical level.
So, in summary, you don't have 15 hours of XP time each week. There is no timer saying that you've played 8 hours, now you're going to gain less XP. You have a limited amount of XP you can gain, both physical and class, each week. Some players will hit the limit very quickly others may never see it.
No matter the outcome, punishing players for playing the way they want to play is not good form. I hope this clears up some confusion about the system as it currently stands.