If you have fiber and the right transceivers, you can run 100Gbit/s over it. If you need that sort of bandwidth, you'll probably do your own peering because just paying for traffic will be obscenely expensive.

Chaos Computer Club's 30c3 conference had a 100G fiber link to downtown Hamburg for a couple of days, still 12k users w/ Gigabit ports and a makeshift datacenter with 10g ports couldn't manage to saturate the link:

https://media.ccc.de/v/30C3_-_5609_-_en_-_saal_6_-_201312301...