The sooner the better. There's no hard and fast rule, but word is the longer you wait, there's some kind of deduction from your overall score. Wait until your trip ends or you have a long layover. Study a bit and utilize CRM when you take the test. Don't get hung up - come back to an involved question if you have time at the end. It goes by quick.

The process here is opaque, but best we can tell, you score points for each phase of the interview process, from your resume to the online test to the phone interview to the in-person interview. Surely some count more than others, but at the end, apparently they're all totaled up into some kind of final score, which determines your ranking in the hiring pool. The pool gets reindexed every time they run interviews, so it's in your best interest to try to maximize your score.