When is the best time to get a valproate level if a patient is taking Depakote DR once daily?  

Levels should be obtained at steady state - which is 5 half lives.  For ER formulation, figure about 5 days and it should be a trough level (immediately before the next dose).

The valproic acid levels are really normed for seizure disorders, not really for psychiatric. As this study shows, if you get a 12-hour post-dose level on Depakote ER, the level will be around 20% higher than IR, so you can just calculate with that correction factor. Or, you can get pretty close to ...

Getting a level just before the next scheduled dose will represent a trough, but may not be practical given lab hours and such. Getting a level 12 hours after the last dose may be more feasible and still useful, but will not reflect a true trough.