The buyer shouldn’t wait more than asked to (seller’s lead time). It’s our duty to deliver when promised.
I’m also a seller and precisely because of this, I have set a 20 days lead time to all my gigs, just to make sure I can deliver to everyone, even though it never takes me that long.
However, I usually tell my buyers the amount of time it will take for me to deliver, which is usually 1-7 days. If something happens and I can’t deliver in 1-7 days, I apologize for it, explain my problem, set a new deadline while still within my lead time.