Your first backup using MzoyPro will likely longer than other backups. This is due to the fact your first backup is the largest since it's a backup of everything specified.
Once your first backup is finished, the following backup will search for files that have changed since the last backup and only upload those changes to MozyPro's servers. Thus on a day off or a weekend this may mean no files have changed and thus there's nothing to backup or upload.
Thus the time to complete your first backup may vary greatly depending on whether your initial backup is 1GB or 10GB.
Additional Consideration: DSL vs. Cable
Be aware that large uploads may reveal the differences between different Internet connections (ie: T1 vs. DSL vs. Cable). If you're like most users, most of your time on your computer is spent downloading information. As most Internet Service Providers (ISP's) are generous with download speeds you may never notice the difference, but when uploading large files the differences may be quite obvious. Speeds vary greatly from provder to provider, but DSL often has better upload speeds than Cable. Additionally, even if your Cable company offers good upload speeds it is sometimes "burstable" in that the first 500MB occur at maximum speed, but then it is slowed down to 50MB/s for the rest of the upload.