Higher frequency seems to give a bit more detail, but less minutes of data. You can also choose to dump data and start over with a new start and basically log one flight at a time. The information icons explain. No minimum runtime needed, but again, if you dump on startup you must download after each run if you want to see data, even if there is room for several runs. I can't remember the exact runtime for recording at top frequency, but about 7 minutes if I recall correctly. If short flights, then you might not choose autodump. If no autodump, you may stop recording when full and not be aware.
I used a short RC servo extension between my timer and esc lead so that I could easily disconnect the timer and hook to the Castle link with that "break point."
Hook up to the link and then view directly on a computer with download. You can then choose to save the data which will create a computer file, and then to later view that file, the graph view. When you select load, you then sees the available files to choose from, those that you have saved. When you save a datalog you will create a title/name for the datalog with your choice of identifiers so that you can select it from a list of other saved datalog files.
All of that said, I turn off datalogging except for when I specifically wish to look at something. Datalogging caused "ticking" and I had some difficulty with the system seeming to have a difficult time deciding which way to turn with the datalogger on. It worked, but seemed to have hesitation. Ticking and the hesitation seemed to come and go with datalogging. Castle suggested that it would not cause harm, but it was annoying and I got most of the benefit from datalogging early on in setting up the system.