I guess it depends on how you measure it. If you're measuring it the way the GB probably is - in dollars and cents - they've probably already peaked. Areas in Europe (UK and Germany) are already showing decline, as is Japan and I suspect Australia has seen their peak. Areas where people have money are not growing. The US has probably peaked too, but they're putting extra effort (the US is now the US branch territory and includes other areas that have some growth yet) into fudging the numbers and picking up immigrants to hide the decline since the US is their crown jewel. They may grow in the 3rd world for some time, but they won't have the economic support that they once had.
I expect them to stop reporting the full stats in the next few years to a decade as they run out of ways to hide the decline without waking up more bethelites with overly dishonest tactics. Honestly, though, I suspect that most of the attrition of the next 10-20 years will be the older generation dying off. The average member's age is way too high for a healthy business, and they'll be forced to reorganize into a leaner cult. The thing that I think will be most interesting to see is if, as the older ones die off, we see an exodus of the next generation as they discover their reason for staying in (often the approval of their parents, it seems) evaporates.