Based on the latest Media Create hardware numbers... (from the week starting August 10)
DS vs PSP: Weekly shares of 70.6 / 29.4 bring total shares to 68.4 / 31.6. If DS stopped selling and PSP continued at this week's rate, it would catch up in 369.6 weeks (September 16, 2016).
X360 vs PS3: Weekly shares of 60.7 / 39.3 bring total shares to 25.0 / 75.0. At this week's rates, X360 catches up to PS3 in 677.2 weeks (August 9, 2022). If PS3 stopped selling and X360 continued at this week's rate, it would catch up in 237.9 weeks (March 9, 2014).
PS3 vs Wii: Weekly shares of 11.1 / 88.9 bring total shares to 28.1 / 71.9. If Wii stopped selling and PS3 continued at this week's rate, it would catch up in 859.9 weeks (February 8, 2026).
Week over week, everything is up, though it's almost unnoticeable for PS2/PS3/Wii. Seems like a holiday bump which these systems have countered by being old/about to be replaced/coming down from another bump.
Through the first thirty-three weeks of the year, overall sales are down. However, the systems can be split into camps of 3 up and 3 down. Here's how the year-to-date year-over-year percents stand as of now.
Home hardware: -36.1%
Portable hardware: -19.8%
Sum of all hardware: -26.4%