The roadside tests in NSW are basically saliva swabs and the results are supposed to be about it being recently enough consumed so as to impact your driving. The stupid part about this is that unlike alcohol which we all agree impairs your driving but since we have accepted it's consumption exists, there is a wealth of information and advice on how to as safely as possible consume alcohol and still drive a vehicle, no such information or guidelines exist for other drugs. This is especially insane as the whole logic for roadside drug testing being implemented was that it was a growing and significant problem, yet not significant enough that anyone in Government or the State regulatory bodies has done anything whatsoever to educate people about how they might as safely as possible do this if they wanted to take drugs and still drive a car at some point. NSW is only interested in punishments and fine revenue and absolutely uninterested in educating or correcting driver behaviour.
I personally know of people who have consumed a range of drugs and gone undetected the next day or even inside 18-24 hours. I don't personally know anyone who has been busted for consumption from a longer time frame than that, but plenty of stories exist of people being busted a few days or even a week later, but no idea if those are accurate or not.
I'd say, like alcohol, the ability for a person to consume drugs and pass a roadside drug test would vary wildly from person to person depending on their body type and physical characteristics. Burgess being an absolutely enormous person who still had a significant enough amount in his system to fail a roadside test suggests to me, without knowing for sure obviously, that his consumption was likely to have been pretty recent indeed.