i buy synthetic urine from the headshop, works everytime. I've used it probably a good 8 or 9 times on urine tests and passed with flying colors everytime. I have, however, failed one because the temperature was too low (i tried using a hand warmer like the one that was in the box with the urine). After that i bought a device called the "urinator" and honestly it was the best investment i ever made- it heats the urine to 98 degrees exactly and stays there the whole time. I've passed every single time with it.
But to answer your question there is a reason why its been stated that 2 days is the normal time to eliminate amphetamines from the human body. If her metabolism is very slow then i couldn't see it being any longer than 4 days honestly.