We don't know what constitutes 'too many', 'too quickly', or how long it takes to cool down--I would assume day(s), not just hours.
It's also possible it's someone else 'nearby' in your network. Some people have had success rebooting routers to get new outside facing IP addresses or using VPNs.
I don't recall if FFF's Browser Cache feature works with AO3 or not.
And worst case, you can always download an epub or other ebook format from AO3, unlike a lot of other sites.