What is the window for settlement negotiations w/ bank and when they charge it off and send it to collection/sue you?

There is no firm written rule. Some will sue as soon as you charge off, others at 90 days. Others might wait 2 years. Each case and each creditor is different.

The fact that they have done a charge-off indicates that the delinquent debt has reached a point where the bank has declared it to be uncollectible, and taken the tax advantage of shifting it from an asset to a non-receivable liability in their accounting books. That provided a tax writeoff. That complicates their willingness to settle, as it would require accounting time to redo their books, and would also require the payment as a receivable asset on their next tax return, resulting in tax obligation.

Additionally, if they have now placed the debt for collection, they have a contract agreement with the debt collector that might govern their ability to separately negotiate on the debt.

And of course, if they have sold the debt to the debt collector, they are thereafter precluded from accepting any payment, as the debt would no longer be theirs.

Too many factors to consider, most of which are factors within the business of the creditor for which the consumer would have little knowledge.

Just try, and see what they say.