It appears from JA's other books that once a gentleman proposes, he must be ready to quit the scene in case he is refused. This is as much to save the lady awkwardness as to protect the male ego.

Notice in P&P that Darcy proposes the day before he is set to leave Rosings, the same day that Col. Fitzwilliam makes clear to Lizzy that one in his financial position could not afford to propose to someone in her financial position. Consider also the awkwardness Mr. Collins gives the family by not quitting the Bennets' after his abortive proposal.

So the likely time for Henry to propose would be just before the family departed Bath--but Catherine was to accompany them to Northanger. Therefore, the next opportunity (unless Henry wished to shortened Catherine's visit with his sister by making her uncomfortable) would be to propose just before she was due to leave Northanger; we shall have to see whether that happens.