At the current rate that its selling, it will take the PS3 3.4 years to catch up to the Wii. This is assuming not a single Wii is sold. But, if the Wii keeps selling at its current rate, it will add about 4,000,000 to its total. So, if both systems continue to sell at the current rate, it will take the PS4 about 4 years.
Of course the systems will not continue to sell at the current rates. PS3 will probably dip more than the Wii, because the Wii simply doesn't have very far to drop. If Wii sales drop to zero and PS3 sales average 80,000, it will take 5 years for the PS3 to catch the Wii. If it averages 50,000 per week, it will take 8.8 years to catch the Wii.
So... I don't really see that happening. The PS3 isn't going to have as long of a post PS4 life as the PS2 did post PS3. The PS2 basically had no opposition. The X-Box and Gamecube were essentially dead once the next generation of consoles arrives. The Wii and X-Box 360 still have some decent life left in them.