Hi,
I think for 11 weeks, an hour of activity time is plenty. Much more and she's likely to get overtired and grizzly.
I would suspect that when she grizzles after 15-20 mins, she could be bored OR frustrated. Like, if you put her in her chair and give her something to hold/grab at, she may get frustrated that she can't keep a grip on it, kwim? If she quietens when you go to her, its more likely to be boredom so when you do whatever you need to do, turn her chair so she can watch you, and sing or chat to her, she'll love this just as much as 'playing' with a toy.
Make sure you watch her for sleepy cues and as soon as you see her getting tired, get her straight to bed, you don't want her bouncy chair becoming a prop!