The key is to write well thought out/ nicely written answers as quickly as you can. You should be spending a couple hours to days max per supplemental imo, since completing your app quickly is crucial for rolling admissions and this is a small part of your app package, however they are still reviewed. What’s good is they actually serve as an advantage a lot of the time, since you end up researching the programs more thoroughly to answer your questions which will help in interviews.