Is there a notebook autosave frequency configuration?

You can set the NotebookAutoSave preference programmatically or via the Option Inspector.

To turn autosave on by default for all notebooks, you could do:

CurrentValue[$FrontEnd, "NotebookAutoSave"] = True

You can also set the option for individual notebooks by changing $FrontEnd to the appropriate notebook handle. According to the docs, NotebookAutoSave saves after every evaluation and seems to be inflexible in that regard.

You could try executing

RunScheduledTask[
    EmitSound[Sound[SoundNote[]]];NotebookSave[EvaluationNotebook[]], 10];

which saves the notebook every 10 seconds and plays a note.

I have a Mathematica package that provides auto-backup functionality. When enabled, the current notebook--call it "blah.nb"--will be backed up to "blah.nb~" after a configurable amount of time has elapsed. I use it constantly and it has saved me from losing work many, many times. It's better than autosaving since it doesn't touch the actual notebook file: if you screw something up or something gets corrupted you don't want to overwrite your main file. :)

It's on GitHub here.