Disney is not giving a firm answer. It was schedule to go for 18 months. From May 5, 05, it should end the begining of Nov. When I emailed and asked in early Feb, I was told they would be celebrating until the end of this year. Others have reported Sept or Oct. end dates. Some have reported that some things will be taken down, but somethings like the fireworks will go a little longer. Its one of those ask 100 people, get a 100 different answer type things! Makes it hard to plan anything.
I have a feeling, based on past experience, that they will spread the word that it is ending in Sept.,then they will extend it a couple times claiming "popular demand" or some such thing.
To be safe I would plan on going before the end of Sept. if you want to see anything related to the 50th.