How long do you have to wait after submitting a marriage visa extension before withdrawing any of the 400K?

Wait until you've got the actual visa approved. Do not let it go below 400K during the "under consideration" period. Was told that by immigration.

Wait until you've made it through the under consideration period and have the full extension in your passport. Once that's done you can use the money.

Just remember that for next year you need the money back in your account 2 full months before you APPLY for your extension. It's not 2 months before your extension ends. So if you want to apply a few weeks early you'll need the money in 2 months before that time.