I always waited four weeks after submitting a script to follow up on a read request. I would tend to follow up on either a Thursday or Friday. The thought process being: If you still haven’t gotten to my script, maybe this friendly reminder will see that it makes it’s way into the coming weekend’s reading pile.
It’s your duty to follow up. Remember, you’re not hassling someone, you’re reminding them that there may be a script in their queue that they were excited enough about at one time to request. These are busy people who have to tend to existing clients first. Just because you haven’t heard back, doesn’t mean they’ve passed on your script.
That said, 90% of the time if you haven’t heard back it’s because they’ve passed on your script. Send one follow up, and then forget about it and move on.