Large tables take hours, if not days. I attempted a test case on AWS using souped up io2 disks (the fastest most expensive disks they have) and a beast of a DB server (r5.12xl I think) and it became abundantly clear that at certain scale you won't be doing any kind of in-place table updates like that on the system. Especially if your allowed downtime is one hour maintenance window per week...
I did it on a r6.24xlarge RDS instance and the CPU wasn't doing much during the operation. IO peaked at 40k IOPS on EBS with provisioned IOPS, I'm not sure if a local disk would be any faster but I already know that rewriting the table and creating the indexes are all single threaded so there isn't much you could gain.
Once I got the logical replication setup to work I changed 20 tables on the replica and made the switch with 15 minutes of downtime. That saved me a lot of long nights.
You can get an idea of how long this could take by running pg_repack, it's basically doing the same thing: Copying the data and recreating all the indexes.
