How much storage should I have for a 90 minute animation project?

Most large streaming services require 4K by default, which is 4x 1080p.

Like @R Neil Haugen implied, a 90 minute project is never just 90 minutes of video space that's needed. That is only true for the final render of the project, but there are typically a lot of intermittent steps to get to that point. The total project size could easily be 10 or 100 times that.

Also, although Photoshop is a great tool for manipulating and now also inventing pixels (generative fill), 2 to 2,5D programs like Illustrator, Animate and After Effects are far more superior when it comes to manipulating vectors. In animation, vectors are usually the name of the game.

Just my 2 cents.

First thing to determine is what your working framerate and framesize will be. Which may be well be more than the deliverable. As I can easily see a deliverable spec of 1920x1080, but needing a working UHD or so.

And the framesize times the framerate times the program length determines what your project will end up being.

You'll need a LOT more room than that. Typically 10 times that just for editing with standard video sources.

Will you have parts you send back out for extra work in other apps? That can also significantly increase project media storage needed. So ... you need to do some calculations here.