I think 5 days will be great to get a feel for how the trial program does, though longer would probably be better, maybe 14 days.

One of VaultPress's strength are the real-time updates which don't waste time and resources by copying and packaging your entire site on a daily basis. If you only get one comment since the last backup, it only backs up that one comment.

Granted, none of this is (probably) long enough to encounter a need to test its restore capabilities, but then again I don't think we should ever hope that anyone ever has to test the restore capabilities.

Disclaimer: I work for Automattic, but was not a part of this particular project.