Cutoff for a paper "in progress" on CV
As an applicant to a PhD program, you want to demonstrate that you have been involved in projects that have lead to publishable results. It is common in my field (let's call it neuroscience today), for junior researchers to list in prep manuscripts. As to what constitutes an in prep manuscript, you should list anything that has a large enoguh scope that you can confidentially discuss the research methods, results, analysis, conclusions and implications. If you can do that, it doesn't matter if you have anything written (although if you are that far along, you should be writing it up).