Based on how much oil your fingers have, the keycaps can be shiny as quick as a month. My Filco spacebar is shiny after 8 months of use.
You can always replace it with another Space keycap. Or use a sand paper.
The shiny keycaps will reappear again in due course.
That's the main reason the new full 104 keycaps are sold to us with 1/3 the keyboard price.