Phosphorylation events happen rather quickly (within mins) but can also decline rapidly. That is why typically you would want to do a range of times such as the ones you have listed to see what works for your system. Also, most reviewers would expect to see moe that a single time point for data on phosphorylation for the main effect.
GLUT and Gsk3 are before Akt activating. (In 1 hour plenty action, in 2 hours normal levels of glucose in blood, physiological), (GLUT make the entrance of glucose to cell).
You must look the Cmax of the drug (plasma concentration), and then 1 to 2 hours. I think. (Depends of the action of drug).