, You can stop the plan after X hours.
Let's say you start plans at 7 PM and you specify "Stop the plan if it runs for 12 hours" and at 7 AM it will be interrupted. Though it's not exactly what you're asking for, this could do the trick.
It works great for file backups since it will continue where left off, but interrupting an image-based (partition-level) plan would require to re-upload everything all over again.

Thanks

Cool, that's exactly what I want. The office location has a pitifully slow internet connection, so it's important that I don't bog it down with a backup.

You may want to look at the bandwidth controls. Rather than stopping the backup, you can slow it down during business hours automatically.

With a max upload speed of 0.5Mb, my avg upIoad is around 0.2Mb, I don't have much to play with.