I would say 25mg to 50mg taken with your hydrocodone, no more. It'll likely stop the itches if you're prone to getting them. Taking over 50mg will probably ruin your opiate high and cause dysphoria.
However, the most common complaint I see with diphenhydramine is RLS (restless leg syndrome). It may cause RLS in people who are prone to it, or possibly worsen the side effect.
But you more than likely won't have that problem.