You can use it right away -- no biggie, as long as the charger is plugged in. And it should take much less than 8 hours to charge it up to a reasonable level. I think it's full charge until about 90%, then trickle charge after that, so the last few percentage points will take a relatively long time.
Historically, Li-ion batteries usually ship from the manufacturer with 33% - 66% charge (much more than that would lead to decreased lifetime)