Totally impossible to give a generic value for. The same splitter on one car will perform differently on another car, let alone different splitters. Variables like rake angle, leading edge thickness, ride height, distance forward of the bumper, diffusers if it has any, etc. etc. To calculate that sort of thing you need CFD data on the specific splitter youre using, and even then you can take those values with a pinch of salt without a full 3D scan of your car.

Splitters can generate massive aero loads though, so they do need to be very strong. The universal one I build Ive done free-air CFD analysis on, and in a perfect situation it will make 400kg of downforce at 200km/h. In the real world thats likely closer to 350kg at 200km/h, but still massive loads. So a 2mm thick carbon sheet wont cut it on a splitter that is actually doing anything!