Manufacturers often don’t publish a camera’s real maximum card write speed, so there usually isn’t a simple official number to look up. In practice, you can use:

• Reviews/tests: Detailed camera reviews sometimes measure write performance with specific cards.
• A practical in-camera test: Shoot RAW in continuous burst mode until the buffer fills and the frame rate drops. Once the buffer is full, the sustained rate is limited mostly by card write speed. A rough estimate is: write speed ≈ RAW file size × fps after buffer fills

For SD cards, the speed labels can be confusing. The older Speed Class system gives a minimum sustained write speed, and Class 10 is generally a safe baseline and commonly affordable. Newer UHS cards are faster on paper, but your camera must support those modes to benefit.

A faster card usually won’t hurt, but buying far beyond what your camera can use may not give any real benefit. For video, use at least the speed class your camera/manual requires; for burst shooting, check review data or test your camera with a known fast card.