How long is a “day” (as at Gen 1:5)?

In this text, a “day” (י֔וֹם or yowm) need not mean a standard 24-hour period, however much some insist on this. Elsewhere in the Bible itself we are told, “a thousand years in thy sight are but as yesterday when it is past” (Psalm 90:4), a verse Peter seems to have recalled when he wrote, “one day is with the Lord as a thousand years, and a thousand years as one day.” (1 Pet. 3:8) Moreover, the seven “days” are recounted on a divine scale, as for most of the time, God is the only actor. It is also a strong and perfectly acceptable argument to point out that, on the most plausible account of the meaning of the events described—an account that attempts to be squared with a respectful, good-faith, scientific observation of the God-made creation before us—the events would quite naturally (but also divinely) extend over billions of years. Given this, the purposes of the sentences seems clear enough: they divide up and order the events using a metaphor.