I wonder how much time you spend per soak to need a TV, but that wasn't your question. Few things will help your estimate:
Where are you? Cold is a factor
How good does it need to look?
How far away from the tub will you want to mount the TV. You wont need a 42" screen if its mounted a foot from the edge of your tub.
Building a wooden box, that's insulated, waterproof, with a glass front that wont fog up isn't that hard. but it'll be hard to make it look pretty. If you're up north, then look into outdoor TVs. You'll be looking at a very short lifespan powering up flatscreens in freezing temps due to internal condensation that'll fry your board in no time.
I did give some thought to installing a bracket to hold an ipad, but it never seemed worth the effort.
