ปริมาณการใช้ต่อเดือน
4
down vote
The to is your main problem. Something isn't convenient to you, it's convenient for you. Compare:
"I would come to your party, but it isn't convenient to me."
"I would come to your party, but it isn't convenient for me."
So "whenever it is convenient for you" would be the textbook answer.
Still, it's understandable contracted as "whenever convenient for you." It would even be understood if it was contracted to "Please call my secretary to arrange a meeting this afternoon, or whenever convenient.