Three weeks.
That was how long they were obliged to wait: three weeks, or three Sundays to be precise, while the banns were read so that anyone who cared might know that when Tom Branson and Sybil Crawley wed, it would be with every blessing law, church, and custom could bestow, and the blessings of family too. Sybil was overjoyed that they could marry with her family's approval, however grudging it might have been, and Mary and Edith had promised to come to the wedding. Still, three weeks sometimes seemed a long time, especially when she considered that they might have been married already if they'd made it to Gretna Green. She was the one who insisted on turning back so she did not bring it up, but she thought about it. A lot.
Three weeks was also a long time to live with Tom's mother. "She thinks we're very foolish," Tom had said, and if Sybil's future mother-in-law was never less than polite, she also never made any attempt to conceal the fact that meeting Sybil and seeing the two of them together had not altered her opinion in the slightest as concerned the foolishness of their union. Sybil, for her part, resolved to be unfailingly cheerful and unendingly polite, and to accept willingly every scrap of advice (housekeeping, childbearing, dress and conduct and manners, though she did admit that Sybil's governess-trained deportment was very nearly decent) that Mrs. Branson chose to give. Tom was also endowed with two sisters, both of them already married. The younger one lived in the country with her husband, but the other one lived in Dublin and visited her mother often. Elaine Murphy was clever and sarcastic and Sybil liked her a great deal.
Tom stayed with a friend for propriety's sake. Once Sybil jokingly pointed out that they had been living under the same roof for some time already, but they had chosen to do this properly in every sense of the word. ("Serves you right," he'd joked in return. "Three weeks is nothing. Try five years.") After a week he moved into the flat that would be theirs when they married. It straddled the line between squalid and shabby but Sybil was willing to overlook that because it was cheap and more importantly it was theirs. (Papa would give her an allowance that was sufficient if not ample, but they agreed that they would rather not live off her family's money unless it were absolutely necessary. It was therefore imperative to rent a flat that could be paid for by the combined earnings of a nurse and a reporter. "Between squalid and shabby" was the best they were going to get, and Sybil would not dream of having it otherwise.)
So they lived apart, as was only proper, but every night he came for supper, and afterwards they went for walks and that was the strangest and most wonderful thing because they could simply be together. She could hold his hand and kiss his cheek on the street and as far as any onlookers were concerned they were only young sweethearts, no more and no less. (He could pull her into an alley or a shadow and kiss her properly, and that was the most enjoyable thing of all.)
Three weeks passed, but they passed slowly.
I'm trying to get through this with as little research/effort as possible, but I must credit Wikipedia for explaining how banns work, and probably all original characters will be named by my choosing at random from among my Irish coworker's Facebook friends.