I ran a few tries, and it does repeat more than one word, which I never quite realized. I think, though, that the following conditions will interrupt the flow for repeating:

• Inserting a paragraph mark
• Backspacing to correct a mistake
• Starting by highlighting existing text and overtyping it
• Any time auto-correct or auto-complete is employed
• For some reason, when I typed an apostrophe (men's rather than mens broke the repeat spell at the apostrophe)

I managed to get a few lines out in one go, but another time realized that auto-correct had perhaps engaged and I hadn't even noticed.

Perhaps try disengaging all auto-correct options in File/Options/Proofing, then see if you can repeat as much as you need. I guess I tend to just highlight and copy anything longer than a word, though if i'm pasting repeatedly, F4 is definitely my go-to.