The setting of this poem is actually an old farmhouse, in which the speaker is laid up, sick on a cot. However, through his lines, we get to do some more extensive traveling. We go back to the speaker's childhood, and before that to Europe during World War II. We get to tag along during his dad's driving lessons. Don't get too comfy, though—Merwin doesn't let us hang out for too long before he's dragging us by the collar to the next spot.
In that way, we can say that the setting for this poem is almost infinite. Now stay with us here. At least, that much is suggested by the amount of times the speaker goes back into the past to add another wrinkle of "ifs" into the mix. We're left to contemplate the long, meandering chain of events that led the speaker to this farmhouse. If anything, this poem is only giving us a few instances that produced him in that moment on the cot. If we really wanted to be thorough, we could stretch all the way back to the first moments of life on Earth because, cheese-n-biscuits, without those, there's no speaker at the farmhouse either.
That's why we say this poem sort of features an infinite setting. The speaker's inviting us to go all the way back to the beginning of time and, if we think about it, we can also imagine the far distant future, too. What role will the speaker play in those events? What role will we, the readers, play? We're all bound up together—no matter where, or when, we are in life.