Here is the link to CCP guide on youtube. It is great introduction to scanning mechanics.
If you want drastically decrease time of scanning Cosmic Signatures here is the the tip:
All cosmic signatures located near moons and can be detected with 8AU probe.
Most of them within 4AU range. I have read some were that all of them within 5 AU and it related to old probes before new Apocrypha scanning mechanics.
Drop 1 probe for each moon and place it on top of the planet. Make probe range 8 AU and scan. You will get list of all signatures.
Screen shots bellow illustrated this in large system (about 60 AU).
All bookmarks are real plexes (mostly GSO) or radars.