How long have the Bennets been married at the beginning of the book?

The Bennets seem to support the adage that opposites attract, as their views and personalities sharply contrast each other. In Chapter 1, we are told that "the experience of three and twenty years had been insufficient to make his wife understand [Mr. Bennet's] character." While he is laid-back, relaxed, and humorous in his parenting style, his wife is scheming, serious, and high-strung. Mr. Bennet also finds his own humor in making witty remarks about his wife's character, especially relating to her primary goal of obtaining husbands for their daughters. She often seems a bit aloof to his wit. Mrs. Bennet takes a very active role in finding husbands for her five daughters; ironically, her own lack of social graces is often a barrier in creating these desired matches. Mr. Bennet is often seen basically hiding from all the women he lives with and providing much more behind-the-scenes support, particularly to Elizabeth.
In spite of their many differences, the Bennets have worked together to raise (mostly) competent daughters and to ultimately have a successful marriage that has lasted over two decades.

Mr. and Mrs. Bennet have been married for twenty-three years. This is truly a remarkable achievement given their apparent incompatibility. Initially, Mr. Bennet was captivated by his bride's youth and beauty. He was also entranced by her seeming good humor. Yet he soon discovered to his horror that his wife was a vulgar, hysterical woman, and so his affection for her began to wane considerably.
For his part, Mr. Bennet is shockingly improvident. He foolishly gambled on the chance that he and his wife would have a son who'd one day break the entail on the Longbourn estate. But as the Bennets have only been able to have daughters, they and their family are headed for the poorhouse unless one of those daughters can be married off to some rich, eligible bachelor. This overriding concern is about the only thing that Mr. and Mrs. Bennet share and is all that holds this loveless marriage together.