It doesn't matter if a backup takes two hours, as long as it needs no supervision. On the other hand, if the backup can't be done when the computer would otherwise be idle, then speed is an issue. Availability is obviously necessary, since you can't use a backup medium if it doesn't exist. Less obvious is the need for the medium to be available even in the future, and on computers other than your own. Otherwise you may not be able to restore your backups after a disaster.
Usability is a large factor in how often backups are made.
The easier it is to make backups, the better. A backup medium mustn't be hard or boring to use. The typical alternatives are floppies and tapes. Floppies are very cheap, fairly reliable, not very fast, very available, but not very usable for large amounts of data. Tapes are cheap to somewhat expensive, fairly reliable, fairly fast, quite available, and, depending on the size of the tape, quite comfortable.