For D&D in particular? I tend to "plan" for about five-level arcs. So, for instance, once I know what the particular game's going to be about, I set up a few situations in motion (evil schemes, intrigues, incursions, etc.) that could theoretically provide an interesting, climactic set piece five levels down the road. So, for instance, if one of the big situations is a malevolent baron who's targeting or has wronged the PCs, then I make notes that he could probably be confronted at (starting level +5) with good success.
After we've done an arc about that length, it's up to the players to see if they want to keep going for another arc or so, or if they'd like to try something new for a bit.
I could theoretically plan for longer (like, say, an entire tier) but I figure it's best not to be too optimistic. Life happens, and I'd like to make sure that there's an opportunity for some closure after a tidy bit just in case.