Here is another snippet which saves all open notebooks on a regular time interval. The best place for it is your init.m file.

1) Use Notebooks[] to get a list of currently open notebooks. 2) Use Extract to filter (hgo) the ones we want ( every notebook except the Messages WIndow) 3) Use NotebookSave /@ to save them 4) Use SessionSubmit to cause this to happen at a given interval

Off[Rule::argr]; (* Suppress an annoying message.  A better way would be a more specific filter  *)  

savePeriod=Quantity[300,"Seconds"];

 obj=ScheduledTask[
 hgo[aza_] :=And[  aza["DocumentType"]=="Notebook" , aza["WindowTitle"]!="Messages"] ; 

NotebookSave/@ Extract[Notebooks[],Position[ Association/@NotebookInformation /@Notebooks[],_?hgo] ] ,savePeriod]//SessionSubmit  ;