If we look at it closely, it is greed that has made Ponzi scheme that disguise as investment close up shop early these days at least some years before we had Ponzi scheme stay for six months even a year but greed when the investors start increasing make the site owners satisfy their greed and leave the site.I don't think that it is time dependent. In my personal opinion i think that it will rely more on the number of people that are actually investing in the website. For example if they have enough investors in order to continuously utilize new investments to pay off old investments and still show a profit for themselves then there are times that these sites will operate for months on end, even with a high interest rate. However, with little support that is when they will see the site only be active for a few days. also it all depends on how soon people realize that it is a scam as well.