The point of no return isn't established by a sales lead. It's established by brand appeal. Depending on how powerful your brand is you can establish it on day one. Take the PS2 for example. By about half a year into its lifecycle it had pretty much established the point of no return. NDS established its point of return with little more than a 5-10 million lead on PSP or when the DS Lite was introduced.
It's not about how much it is over the other but how much appeal it has garnered. Because if you are the dominant brand then you wil continue to sell at all times while the others will fade in certain times of the year.
I'm not sure Wii has fully established this yet but I think its well on its way and may be there by the end of the year.
