I wouldn’t say a “random” amount of time - it happens whenever Panda is rendering the geometry holding the texture, which is when it passes it to the GPU. You might be able to force this to happen earlier using Texture::prepare(), but this is not what setRamMipMapPointerFromInt was designed for in the first place. It was created by someone who needed very low-level access, and the method should be avoided unless you’re sure you need it and can guarantee that the data will be around for long enough.
setRamImage/modifyRamImage is usually the method to use for purposes like yours.