] When would Henry have proposed if his hand had not been forced by the General's bad behavior?

At the end of Catherine's visit to Northanger Abbey. The invitation there would raise expectations, and to some extent it's a period of getting to know each other and ensuring that the other has no qualities that would make them an unsuitable partner -- it's a courtship period, and that seems pretty well understood by all the Tilneys.

The time to resolve the matter would have come at the end of her visit, because to propose before then would leave open the embarrassing possibility that she might refuse, which would make her continued stay embarrassing to all concerned.