The lifespan of a keyboard even a Logitech keyboard will depend on how much typing you do on the keyboard and how hard you pound on the keys.
Logitech makes good computer keyboards though and I have a Logitech Keyboard that is 5 years old now and although the letters have worn off the keyboard still works like it did when new.
I don't really need the letters anyway since I type without looking at the keyboard so the letters don't matter to me.
But as long as you use your keyboard like it was meant to be used then you can expect at least 5 years of use out of it and sometimes longer.
Usually you'll be getting a new computer before your keyboard needs replaced and new desktop computers come with new keyboards anyway.