It's been a while since I saw the last few episodes (and the first ones for that matter) but, if I remember correctly, before Dorothy gets married she says something to the effect of "It's been a great seven years." And, yes, that's how long the show had been running. But when the show first came on, they'd been living together for a few years already but Sophia was just coming to live with them (Shady Pines burned down) and Blanche was on the verge of being married.
I was also confused about the fact that even though each of the ladies was from somewhere else, they all wound up in Miami. It seemed at times that Blanche raised her family there but I could never figure out how Dorothy and Rose came to live there. Rose and Charlie didn't live in Miami because when the girls mistakenly thought Blanche had slept with Charlie (due to a double exposed photograph from an old camera of Rose's that made it appear Blanche and Charlie were in bed together), they assumed that Blanche met him because he was a traveling salesman who traveled to Miami. So clearly Rose and Charlie still lived in St. Olaf when they were raising their kids. Also, none of them have children or family who live in Miami (except Sophia). When Dorothy's friend from college came to visit, she had a party and invited people who knew Trudy. So does that mean Dorothy went to college in Florida? It's all very muddy.
Ah, who cares? I adore this show.