It's been said already but... it's been, at the most, one year if you go off the earliest release of PH. Nintendo, in nearly every interview, says there is usually 3 - 5 year intervals between each release. So, if the DS one came out a year ago, the earliest you could expect a new one is in 2010/2011. The Wii, on the other hand, is due a new game roughly the end of next year or so.

And, the fact you think having a engine means making a game is simple is beyond me. It's not just about making a game, it's about making a game that doesn't suck. The next Zelda annoucment will be for a Wii Zelda, that much is sure. The debate now is that it seems the next Wii Zelda could be First person based off speculation, LCT, and the Fact Myiamoto said in a japanese interview that he wanted OOT to be FPS but it wouldn't work well with the whole "aging" aspect.