Welcome to the forum. Your questions are quite impossible to answer, I'm afraid. Depends on how good you will become and how complex the terrain is going to be. I have quite some experience, but sometimes struggle to get something complex done. It also depends on what you want to add as imported objects, whether you need to put many objects at specific places and angles, or if it's 'just' some populations of trees.
Render time is also quite dependent upon ingredients of a terrain. Much (rough) water and displacements, many complex, heavy objects, clouds (especially easy or v3 clouds) take up the bulk of rendertime. And the required quality plays an important role.
I would just start and see what happens when you build your first landscapes.
