I believe it will be sooner than many of us perhaps think, in the western world at least.
I noticed in a thread the other day about baptism that the longest most people raised as jws waited to get baptised was the age of 18, and most got dipped well before that age. The last few assemblies I attended, just about all the baptismal candidates seemes to be young and even pre teen kids of jws who had been pressured into baptism by their parents and congregation elders. I can hardly remember the last person to come in to my old cong through being contacted in the field ministry, unless it's happened in the 18 months since I stopped attending, which I very much doubt. The last one I remember was in 2001, and she had become just about inactive when I left.
Obviously, the wts must be aware of this lack of success in the ministry, and they must also be aware that there is little or nothing they can do about it. The few who might express any interest in what the jws have to offer these days only have to log on to the internet to investigate the religion for themselves. I would think that anyone doing so, and learning what it's really like to be a jw, would lose interest very quickly. No wonder the wts hate and fear the internet so much!
I believe that in the very near future, any converts from the field service, particularly in the West, will dry up altogether, and the wts will be left with only the children of other jws to baptise. I have known kids as young as 8 or 9 being pressured into baptism by their parents, with the full support of their congregation elders and sometimes even the circuit overseer, and I am certain this trend will continue, with the pressure being exerted on even younger kids. Heck, they might be baptising them before they can even read soon. It's the only way they will be able to sustain any kind of growth, or more likely arrest the inevitable decrease the wts is likely to suffer over the next few years. They have always spoken against infant baptism, but I wouldn't put it past them to come up with some "new light" to make it ok to do that, if numbers really begin to fall.
I doubt the wts will crumble altogether in my lifetime, much as I hope to see that happen, but I believe that in the next 10 years or so it will suffer a significant reduction in membership compared to the current membership figure.
Linda