It depends on a lot of factors, but my preference is at least 450-500 yards (411-457 meters) of yarn.
You can read about some knitting yardage rules of thumb in “Shawls to Play With.“
Can you knit a shawl with less yardage? Yes.
“Wafian” is knit with 420 yards (384 meters) of Tosh Merino Light – a fingering weight yarn – on a US 7 (4.5mm) needle.
However those shawls tend to end up on the smaller side. More scarf-like than shawl-like. But I like my shawls on the giant side of huge.
That being said, the sky is the limit with shawl yardage.
“Tumbling Deco” takes 980 yards (896 meters) of a lace weight yarn.
“Eirwen” takes about 1600 yards (1463 meters) of lace weight.
“Ingrid’s Baby Bridal Knot” (one of the first lace shawls I ever tried knitting – it didn’t go so well…) calls for 3825 yards (3498 meters) of lace weight yarn.
And “The Queen Susan Shawl” calls for 6562-7108 yards (6000-6500 meters) of yarn. I’ve never tried knitting this shawl, but it’s the largest I could find with a quick search in the Ravelry database.
“Modern Heirloom” is a close second with 5000-6000 yards (4572-5486 meters) of yarn.
So the sky really is the limit. You can knit a shawl as large as you want, with as much yardage as you can stand. What’s the largest shawl you’ve ever knit?