It's hard to say how long Watchtower will be around. When you're talking about an organization like the Watchtower, you're talking about a system that is comprised of many smaller systems within it.
Additionally, you have ever changing uncertainty to deal with, as well as emergence of larger forces due to all those smaller variables.
You also have unexpected consequences to deal with and couple that with exponentially increasing computer/technology power and you have a recipe for some interesting probabilities.
Technology is the equalizer. It equalizes the power and the playing field. Now the average person can cause a ripple effect of positive change.
Most people don't really understand Moore's Law of exponential returns. Computer technology does hot advance the same tomorrow as it did today. It doubles the doubling of the doubling that was doubled by the previous doubling. (Example: So what used to take 20 years of progress in technology now takes a day. Tomorrow that same 20 years of progress will only take 12 hours. The day after that 6 hours. ) (The Age of Spiritual Machines. by Ray Kurzweil)
Everything has a life cycle and that includes religions/cults. One thing for sure...the organization has been forced to make change. Change is the crucial key. Although many of the changes have ranged from ridiculously lame to completely bizarre....it is still *change* that the organization has been forced to make. There is an interruption to their status quo and now it seems like they are in hot water financially.
I personally don't think it will even take 5 years for them to completely die out as they are now. There may be things within the organization that none of us know yet, that if we knew would change our opinion about the time frame to Watchtower's self-Armageddon. However, this is just my guess in the dark with absolutely no concrete evidence to support my theory/opinion