Let's see... typically sales are about 100-150k per week for PS4 and about 40-50k for XO, with either one spiking to around 200k when there's a major release. There aren't any more big releases for either console until September (Destiny arrives on both consoles, though that one seems to be getting marketed more for PS4).
If they keep up about 50k and 125k per week through the next few months, then by the beginning of September PS4 will be at around 9.4-9.5 million and XO at about 5.1 million. I have a feeling both might sell less than that, though; this is typically the slowest sales period of the year and with hardly any games coming to either console, I could see both of them dragging a bit.
Anyway, it doesn't look like it'll happen any earlier than September. Since Destiny is coming to both consoles and will probably spike sales for both a bit, I'm actually going to say that it won't happen until the end of the year.
