Three years elapse from the opening scene to the final chapter. The story's drama heightens and continues to compound throughout the summer and fall that Margaret is nineteen. A staggering sequence of trying events occurs in these few months.
Here's a helpful timeline of events I've put together from the scribbled notes and underlinings in my well-worn copy of North and South.
North and South Timeline
July - Edith's wedding in London (Margaret is 18); Margaret returns to Helstone.
mid-October - Henry visits Helstone; Mr. Hale announces his decision.
November - Hales arrive in Milton; first meeting with Mr. Thornton.
"early spring" - Margaret meets Bessy; Mr. Thornton comes to tea.
June - Margaret is 19.
--- about a year has passed since the opening scene ---
July (or August)* - Thornton dinner party (Thursday, the 21st).
3 days later - Riot at Marlborough Mills.
day after riot - Mr. Thornton declares his love; Margaret visits Bessy for the last time; Margaret sends a letter to Frederick.
second day after riot - Bessy dies.
October ** - within a space of about two weeks, the following chain of events occurs:
Frederick arrives; Mrs. Hale dies; Frederick departs, leaving Leonards injured; Margaret accompanies her father to her mother's funeral; the police inspector questions Margaret; Thornton saves Margaret from the inquest; Boucher dies.
a few days later - Margaret asks Higgins to seek work with Thornton.
the very next day - Mrs. Thornton comes to chastise Margaret; Higgins asks Thornton for work; Margaret visits Higgins' home; Mr. Thornton takes Higgins on; Margaret and Mr. Thornton meet in the street and Thornton gives his "foolish passion" speech.
November - Mr. Bell comes to visit in Milton and notices something between John and Margaret.
Winter months - Hales fall into "the monotony of [a] quiet life" while Mr. Thornton is busier than ever at the mill.
April - Mr. Hale goes to Oxford and dies in his sleep; Margaret leaves Milton.
June - Mr. Bell takes Margaret to Helstone. (Margaret is 20.)
--- two years have passed since the book's opening scene ---
August - Mr. Bell dies.
September/October - Margaret and the Harley Street residents vacation at the seaside in Cromer.
November through June - Margaret carves out a life of some purpose in helping the poor in London; Thornton tries hard to save his business while the financial market begins to fail.
Early summer - Mr. Thornton is forced to close his business. (Margaret is 21.)
--- three years have passed since the book's opening scene ---
"A hot summer's evening"- Mr. Thornton comes to dinner in London. (It has been well over a year since John and Margaret have seen each other.)
the next day - Margaret meets with Henry Lennox about her desire to help Thornton.
two days after the dinner party - Margaret offers Thornton her inheritance; the remaining barrier of misunderstanding between them is dissolved.
Notes:
Unfortunately, Gaskell wasn't always accurate in keeping record of passing time. There are a couple places where things get a bit muddled and can't be outlined in an exact timetable.
* A reference to Edith's wedding one year ago suggests that the Thornton dinner party is July 21st. But when Margaret walks to Malborough Mills a three days later, Gaskell describes the heat of an 'August sun.'
** References to the passage of time gets very difficult to track after Bessy's death a couple days after the riot (July or August). Frederick appears to arrive much too soon to be plausible considering the modes of communication and transportation available. And after Mrs. Hale dies a day later, we suddenly find ourselves in October!