Estimate amount of downtime from RDS backup
Usually, RDS backups don't cause downtime. Similar to EBS backup, the I/O needs to freeze the DB for a split fo the second to get a consistent LVM snapshot. After that, it is irrelevant how long the backup takes. Alternatively, you may consider taking the snapshot on the Read Replica to avoid the impact on Master.


  During the automatic backup window, storage I/O might be suspended
  briefly while the backup process initializes (typically under a few
  seconds). You might experience elevated latencies for a few minutes
  during backups for Multi-AZ deployments. For MariaDB, MySQL, Oracle,
  and PostgreSQL, I/O activity is not suspended on your primary during
  backup for Multi-AZ deployments, because the backup is taken from the
  standby. For SQL Server, I/O activity is suspended briefly during
  backup for Multi-AZ deployments."
  https://docs.aws.amazon.com/AmazonRDS/latest/UserGuide/USER_WorkingWithAutomatedBackups.html


Since you've mentioned that you need to perform RDS Maintenance after the backup.
you may want to check out how to avoid downtime altogether during the RDS maintenance.
https://workmarket.tech/zero-downtime-maintenances-on-mysql-rds-ba13b51103c2

Disclosure: I work as a Lead DBA for WorkMarket.