How long can I leave the tree is leaning now and it wasn't before before it becomes dangerous?

The failure will most likely complete during the next wind or rain event rather than on a calm afternoon, so the practical deadline is 'before the next storm' — days, in an Indiana summer. Rope off everything within the tree's height plus a third right now, and keep vehicles out of it. It escalates to immediate if you hear cracking, if the lean visibly increases hour to hour, or if the soil mound is growing. Conversely, a lean that has been there for years with completely undisturbed soil at the base is a watch item and not worth a night rate.