How do I stop being my friend's default manuscript reader without withdrawing my support?

My friend arrived for lunch, put her laptop between us and said, "I've fixed chapter six. Can you read it while I order?" I'd wanted to tell her about a weekend away. Instead, I spent the first twenty minutes looking for the changes she'd made.

She's writing her first novel. A few months ago, I offered to read an early chapter because she was nervous about showing anyone. I enjoyed it, and I told her I'd love to know what happened next. Since then, she's sent new chapters, revised chapters and questions about every comment. I usually answer, so I can see why she thinks this is an arrangement.

I am interested in her book. I'm less interested in opening the same chapter for the fourth time when we'd planned lunch. She says my feedback helps because I know her and will be honest. I don't want her to hear 'I can't keep doing this' as 'Your book isn't worth reading.'

Should I offer a smaller, definite amount of reading, or stop for a while altogether? How do I ask for our friendship back without sounding as if the thing she's excited about is an inconvenience?