They will last between 2 to 8 weeks, so you may need to be prepared
for some odd behaviour
for a while. The drunk-like state needs careful supervision, particularly on the stairs, as her coordination is severely impaired and will be so for a while. Once the "therapeutic level" of phenobarbital has been found (it's a bit trial and error), the side effects should start to dissipate.
Of course, the question is why did the seizures begin?