It’s been a while since I tested but I recall it being non-linear. Sometimes my estimate were off by hours, other times weeks or days. I never bothered to figure out what steps caused the less accurate predictions.
That being said, it’s possible it could take that long - though unlikely unless you have a very high volume size and/or slow connection to your destination AND some bad dindex files (forcing downloading of the large dblock files to get the missing info).
It’s more likely that the number and length of file paths in your backup (rather than number of volumes slows) down the rebuild time.