A lottttt!
Better to push back and ask on the face when given an assignment. Startups do this as they get free work done. Established companies might do it less.
Also if they don't address your concern, it's a red flag. You can do a screenshare of your past assignments and show them.
Marketing/Growth/Product etc have it easy to steal. Finance assignments etc less so. Depending on what they are asking, if it's a random case study, do it. But if it's gonna be useful for the firm, take your stand. They will hire a junior with half your CTC and ask him to execute your submitted plan.