We've never imposed any limitations on DD6's screen time, from the time when she was an infant, and her attention span is awesome. It always has been, even from infancy. We were amazed that a 4mo could watch an entire 20 minute show for preschoolers.
Of course, the major reason it's not being regulated is because she's not abusing it. She'll spend some time watching a show, playing on her DS, etc., but it's not something she wants to do all day long. She recently told us her favorite thing to do is read, and when she has to self-entertain, she spends more time with her toys than she does in front of a screen.
It sounds to me like your main concern is that your DD isn't getting enough 1-on-1 time in her day. There are a number of issues that can develop from that, but one of them would be communication delays. Television can help with that, because while she may not be participating in verbal communication, at least she's being exposed.
I'd expand her choices beyond sign language DVDs.
