Tom Petty had it right. The waiting is the hardest part.
As a former magazine editor, I can say with certainty that most writers on the submission side of the process truly have no concept of how many manuscripts land in an Inbox. It's a colossal number, and that's from the perspective of a small publication, not the biggies.
Whether we're talking about literary magazines or trade pubs/websites, or full length manuscripts: editors are overworked and quite often not paid particularly well. It's a hard reason to hear as a writer desperate to get an answer, but it's almost always the reason for the wait.
Image: clock
If enough time has elapsed (and enough varies depending on context), you can (and should) follow up.
In the meantime, keep submitting. Keep writing. Do everything you can to stand out in the slush pile by grabbing — and keeping —attention.