You'll know when it's time to re-hone. Just like you know when it's time to toss your DE blade. When it starts to pull/tug and just doesn't feel as sharp anymore as when it had a fresh edge. There is no real time-frame as there's too many variables. How many times a week do you shave with the razor, your beard type, technique,... Someone with a very coarse beard will blunt the razor faster than someone with a light beard. If you only shave with the SR once a week you could easily shave for months but if you shave every day then it won't last as long. Bad technique in shaving and stropping will also put more strain on the edge.
Some like to touch it up and refresh the edge when it slowly starts to pull until it needs a new honing session. Others just reset the bevel and completely re-hone the razor without wasting time. There's no wrong or right. Whatever works for you. When the shaves becomes uncomfortable it's time to take it to the stones regardless.
"Simple: not to be confused with easy."