I would say "When did you mow the lawn?"
A: I have mowed the lawn.
B: When have you mowed the lawn?
(Quoted from Illustrated English Sentence Structures by Su Qin, p258)
Is B's question in the above dialogue acceptable?
I need native speakers' help.
It's not very natural. You might hear a variant on it in a dialogue such as this:
Mum: What are you going to do this weekend?
Kid: I'm going to do my homework.
Mum: That's great. What else will you do?
Kid: I'll probably go to the cinema.
Mum: OK. Are you going to help me with any chores?
Kid: Ummmm, I might mow the lawn for you.
Mum: Hah! When have you ever mowed the lawn?
Remember - if you don't use correct capitalisation, punctuation and spacing, anything you write will be incorrect.