A good friend of mine says he can use kratom chronically and then switch to suboxone right after dosing and it is no problem and he has done this many times. Personally I have always waited until I feel the withdrawals, I try to wait a whole day but have probably done it after 12 hours.
I am thinking that because kratom is only a partial agonist, suboxone, also being a partial agonist, will still knock the kratom out of the receptors, but will satisfy your needs fully, unlike with full agonists, where a partial agonist will knock them out and then due to being a partial agonist, will not fulfill the needs of your receptors which have become dependent on full agonism.
But it's best to be safe and wait as long as you can. If you're feeling the full withdrawal you're definitely good. I have in the past waited for the first signs of withdrawal (watery eyes, some anxiety), and then waited a little longer until I started to get restless and uncomfortable and more anxious, and then taken suboxone, and it makes me feel good as soon as it takes effect, no sort of difficult transition like with full agonist opioids.