Great question and a wonderful answer.
Answer: whenever your book is ready!
The only time you shouldn’t query an agent is when they’re closed to queries, and some agents never close. (Note: PSLA closes from Christmas to New Years Day.)
Don’t waste your time waiting for the ‘okay’ to query.
Some people say don’t query during certain months or holidays, but agents read all queries, and generally in the order they’re received. So if you query on a Sunday night we don’t read it on a Sunday. We’ll read it at noon the next Thursday.
Don’t give yourself an extra excuse to get that project out the door!
Q: What excuses do you tell yourself to stop you from querying?