There’s no real formula as PRDs depends entirely on maturity of product/feature. How to solve then? 
  
 Communicate - Once you get task to write PRD, ask for 2-3 days to provide an estimate. Write down all the potential areas which you would be including, put line items in scope/out-of-scope. Discuss with your engineering counterpart the order of releasing stories/PRDs. Once this alignment is done, share this plan with your product leadership. Let them go and fight the battle(s) with the CEO.

Product Manager

My god! 
 This sounds like a terrible product culture

Senior product manager

Don’t set unrealistic expectations for the releases

Associate Product Manager

Last week, I got a list of 10 features to be implemented. I have no research on the features. But I have to write the PRDs for all of them. How do I communicate

Last week, I got a list of 10 features to be implemented. I have no research on the features. But I have to write the PRDs for all of them. How do I communicate to my manager, the ETA to write the PRD. Since every feature could require different research time, different scope. 

Also the CEO is imagining this project will be delivered in next 1 month.

How to estimate time to write a PRD?