Lets see, I guess whenever I have the urge and the price is right. I don't follow any predetermined set period of time that I'll wait to get a system, and I'll never camp out on launch to get one as I have plenty of other things to do.
Anyways, Here is when I got most of my consoles.
NES - gift for my 5th birthday after I ate Nintendo System Cereal and told my parents I want one.
SNES - Gift for Christmas the second Christmas it was on the market.
Genesis - Gift for my birthday, I think after it had been on the market for 2 full years.
N64 - Worked all sumer to buy one, only to find they were all sold out shortly after launch. Luckily, my parents decided that since we worked so hard over the summer they found one for us for Christmas.
GC - Bought shortly after launch with my 2 brothers.
PS2 - After DDR came out and my friend brought over Burnout 2 for the GC and I fell in love with it and I saw that Burnout 3 was out.
Wii - Bought one in late January 07.
EDIT: forgot the PS2
