Once you get into this sport, it no longer becomes a matter of "how many days" but "how few." Most of us would ski every day of the year if we could.
Sunday River (SR) has more than enough terrain to keep a group of skiers busy for a week. There's also a lot of slopeside lodging, and some good learning and kids programs. Outside of skiiing, however, SR has limited offerings. Bethel is not as well-equipped with shops, spas, and restaurants than say, Stowe, Vermont. As people at DCSki know, Stowe holds a special place in my heart. But Stowe is not for everyone--especially lower intermediate skiers. SR has much more terrain for this type of skier. It's also got some decent upper level intermediate terrain and even some challenging expert trails. Families who just want to hang out and ski love SR and go year after year. It's 3.5 hours from Boston if you wish to spend some time there and about 12.5 hours from DC give or take. Stowe, by contrast, is about 11 hours from DC.
One resort that might fit your group is Killington. It's huge with lots of benign intermediate terrain. Woodstock is nearby for those who'd rather spend a day shopping and seeing a quintessential Vermont Village. Killington is also a fairly easy drive from DC-about 9 hours. There are many good restaurants in the area and a legendary après ski scene. It is big and can be intimidating but once you figure the place out (which takes 2 days or so), you start to enjoy the mountains a lot more. It might be worth taking an orientation class at Killington for your first day. Also, don't miss Pico.
Okemo and Stratton are also good intermediate mountain, although I've never visited them so I can't speak from personal experience.
One last word about Stowe. My father was a consummate advanced beginner and loved Stowe. He was happy to ski Toll Road the entire day because of its length and diversity. Stowe's Spruce Peak area also has some easy transitional blues like Smuggler's and Sterling, but Spruce has a southern exposure, so there could be a snow problem there late in the season. The problem with Stowe for the Mid-Atlantic intermediate is that their signature blue trails--Lord, Perry Merrill, and Gondolier-would rate as black trails in this region. You don't see double fall lines on most Mid-Atlantic blues.
Also, most of Stowe's double black trails are well beyond the capability of a "Mid-Atlantic" expert. As I've said before, I'm still trying to summon the courage to ski Goat. And then there's the off-piste at Stowe, which is extreme by any stretch of the word.