I love reading about this subject on the WTM Boards! It's amazing how different people's opinions are! Currently, my thoughts on seat work are changing. I firmly believe that young children need to learn through playing, drawing, and letting them run around freely. But I also believe that once they get to a certain age, they need to learn to be able to sit down and pay attention.
I recently read a quote that I really liked and believe. It states that "absent discipline equals absent learning." Now what they want it to mean, I'm not sure. But for me, it does mean that if your child is used to being squirmish and non-attentive, then that's going to make it that much more difficult for you to teach them. Eventually, all children need to be able to sit down for a certain amount of time without being constantly distracted. Once they come to kindergarten age, children should really be able to sit down for at least 15-30 minutes. This means that you can read them a book for that long and they won't get up and run away or start doing something else.
Of course, all children are different and I'm sure that other parents' experiences differ. I've slowly worked my way up with Marco and now he can last about 45-60 minutes on certain things. Examples would include sitting at Mass on Sundays, going to a movie, and on the more academic side, being able to sit through a math lesson. If after 20 minutes there comes whining and complaining, then I'm the first to say "let's take a break." But if after only 5 or 10 minutes there's whining, then I don't bend so easily.
It also seems to set a great example for Isabella, who is constantly asking to do "school" with Marco. It's fun to include her, but she still has a problem with following directions. I can't tell her to do something and then have me leave. I have to stay with her to guide her and direct her...which is a given, since she is only 3 years old. But I have always seen a big difference between her and Marco at this age. Maybe it's her personality or maybe it's that she's a girl, but is so more willing to please and listen.
Anyways, as the summer months approach, I will be making sure that Isabella gets some good seatwork along with big brother. I'm aiming to start with 1 hour total in 10-15 minute increments. I'm sure that by the end of the summer she'll be ready to jump into her own little "school" schedule and I know that she will be all the happier for it!