Forever - I just got off the Miracle about a month ago. I had a balcony room (7196) and can't exactly remember if there was a grab bar, but I do have a picture of the bathroom at home, so I can check for you. What I do remember, and I'm not sure if this will help you at all, is that the sink was very close to the toilet - that way, you can use it to prop yourself up if you need.
RCCL - Adventure OTS (Honeymoon!)
CCL - Carnival Miracle
RCCL - Enchantment OTS