When does fluid resistance switch from F=kv to F=Dv²?

[SecretSnow]

Hi all, I know that for low speeds, the fluid resistance is F=kv while at high speeds, it's F= Dv^2. However, how do I ever know what is defined as a low speed and high speed? This kind of explanation is so qualitative and I think a quantitative one is better. Thanks all!

 

[mfb]

You can use the Reynolds number to get some estimate.