De vez en cuando

Por algún tiempo

en algún momento

In addition to Iago's comment, the word "somewhen" is an archaic form of "sometime" which in Spanish would be rendered by phrases like algún día, tarde o tempanto, en algún momento, un día de estos, etc.

Originating from the Isle of Wight,(U.K) Somewhen is a combination of somewhere and sometime .It can mean either or sometimes both - Used casually it often means never as in We'll meet again don't know where don't know when but I know we'll meet again etc. We must meet up together somewhen ! (From Urban Dictionary) So I think something like "cualquier día'', "por ahí" would work