As common as the use of bows. Which one is chosen depends on what one needs a range weapon for.

[To guard a fortress, archers. To kill a single man, a crossbow.]

I wouldn't call myself an expert at the crossbow either, but I know enough that sounds like a fair trade.

[He is curious why they're uncommon in Claude's homeland, though, even if the reasons are purportedly boring. A crossbow is useful and uncontroversial in his experience.]