Depends what your device can handle. Think of it like this: the full model might be 4Gb, if your device has 8Gb it might fit in memory, so 100% of the layers can be loaded there (and still have some room for the system and apps and such). But if your device has only 6Gb or 4Gb, the whole model will not fit, so you will need to test if 50% can be loaded into memory, or maybe less. At some point it might not make sense to use the GPU if only too little layers are loaded there, since the overhead of combining CPU + GPU work can predominate. Also, you need free memory space for the context window, so bigger contexts will consume more space and leave less space for layers, while smaller context leaves space for more layers.