Hi KD,
Vyvanse and most of the stimulant ADHD meds work fast (that day) and wear off fast (that day). I'd say you'll notice it right away, but it may take a few days for you to see enough evidence of the up and down sides to know whether it's right for your child.
That particular med ramped up my kid too far (it was like several cups of coffee-- really fantastically focused, but also on a hair trigger and tipping over into overemotional some of the time). After trying several stimulant meds we ended up using a non-stimulant one which he tolerated better.
DeeDee
