How does she feel afterwards?
That's something I've used as a metric with my kid (almost 10). She's taking ballet, a really rigorous, disciplined type of ballet, and it's a serious challenge for her. I think challenges are good for her though. When she started this year it was frequently really tough to get her to go to class, but then afterwards she'd rave about how awesome it was. She had a sense of accomplishment once she got through it, but beforehand she wasn't looking forward to the effort.
Because she's so positive about it afterwards I went ahead and pushed her beforehand.
That did ease up though as the year went on, now she actually looks forward to it. I could have easily given up on pushing her when she was complaining so much in the first couple of months.
Which is not to say that pushing is always a good idea, though, at all.
One other thing that helped was just making sure she knew exactly what to expect, schedule-wise. I think part of the difficulty was that I'd remind her an hour before she had to be in class, when she already had other plans (if only to lay around reading a book). Once she locked in that at a certain time on a certain day she was going to ballet, there was less resistance.