I am off of keppra (levetiracetam) but the rash continues.. how long should it take for the itching and rash to stop?

Depends: More information about this is needed, but I suspect you're describing a delayed drug rash related to Keppra (levetiracetam). Multiple potential causes for such a rash exist, and depending on the scenario, the rash could persist for days to weeks. If it is improving, continue to closely monitor. If worsening (or fever, low urine, abd pain, bleeding/bruising), seek urgent care even though you stopped Keppra (levetiracetam)

A couple of weeks: the rash and itching are known dude effects of the drug. Contact your doctor if you still have these symptoms beyond a couple of weeks. Main question would be to get an alternative medicine. Feel better:)