Ok a lot of people have been talking about when to use it etc and i had been thinking of a way to work this out for when it would be the best time, now that the race is only 80-90 it reduces your options quite a bit, the way i see it you can do as follows:
Use all 8 hours for 80-85
Use all 8 hours for 85-90
Or split them between the two
Now i haven't leveled to 90 on the beta (got half way and stopped) but people were saying it was stupidly short and that you could do it in 10 hours or so? and as expected they increased the exp needed by 55% which in theory would mean it'll take you around 15- 16 hours, as i said i haven't done much leveling on the beta so don't know how accurate this is but it doesn't make much difference, you can take the time to level in this post as examples..
Now that the 82-85 has been reduced by quite a bit this makes leveling to 85 a lot quickier, lets say you can ding 85 in 8-10 hours
Thats 8-10 hours for 85 and 15-16 hours till 90
As long as the leveling from 85 - 90 is longer than the 80-85 i see it more efficient to use the 50% extra bonus for the 85-90 stretch due to you gaining a lot more exp and getting more benefit out of the exp bonus, also as it is the mop leveling zones flow together quite well so there isn't a lot of zone switching where as if your using it at the 80-85 stretch then you could waste some of the buff