Google regularly runs these at 100%. According to the B4 paper:
"""These features allow many B4 links to run at near 100% utilization and all links to average 70% utilization over long time periods, corresponding to 2-3x efficiency improvements relative to standard practice"""
Well, you wouldn't want to hit 80% on Day 1, as you would have no room for growth. Perhaps 50% and after a few years† you'll hit 80% and start planning for a new cable.
† The video said this started five years ago, so there appears to be a lot of lead time that is needed.
I remember seeing a cloudflare post about AWS bandwidth pricing where they estimated something like 20% utilization? I don’t remember where though but I think they can approximate pretty well.
"""These features allow many B4 links to run at near 100% utilization and all links to average 70% utilization over long time periods, corresponding to 2-3x efficiency improvements relative to standard practice"""