When I can write to the sprite memory ?

Always. If the CPU tries to access sprite RAM at the same time as the sprite hardware, the CPU just waits until the sprite datafetch has finished. The more sprites in the scanline, the longer the CPU has to wait.

Are there any scanlines where sprite hardware datafetch does not happen at all ?

Yes. During the full border scanlines and any scanlines on which there are no visible hardware sprites. On such lines, the CPU can write to sprite memory with no wait cycles.