This question seems to come up once every other week or so. Like the late Mr. Petty said, "The waiting is the hardest part."
It's really tough waiting those two to four days, especially when we think we have completed something brilliant and are eager to share it with others. I'm in that spot right now with two more poetry pieces that have been pending since July 26 and 28. There haven't been any new erotic poetry postings since the 28th, so I suspect the poetry editors are on holiday or there are larger website projects demanding everyone's time.
I'm on my fifth day waiting for my (reedited) story to post. Eight if you count the original submission, rejected by the moderator. My experience says stories post three to five days after submission. Maybe longer.
There are number of factors, or at least there were last time I submitted. 2-4 is average as everybody has said but I've lucked in to 24 hours more than once with short stories.
The longer you get or the closer you are to violating the rules the longer it takes. Like even though there are plenty of twenty somethings who wear their hair in pig tails. If you are writing a high school story you need to shy away from anything that would imply the character is young since you are already playing with fire. She does not have a favorite stuffed animal, not even Tender Heart Lion her boyfriend won at the fair and had to carry for her because it was bigger than she was.
So, for my first story, it was 8 days. My second is 4 and counting.
I don't know if I have just had bad luck or what. But from the moment my first story went to the editor until it was posted was 23 days. I had wanted to get it post in early July, it ended up late July. My current story was to the editor very early August and I expect it to post sometime early next week. Since I posted in Taboo (1st story) and LW (2nd), that may have something to do with it. They are 2 of the "Big 4" I think for submissions, which means they may take substantially longer to go through the posting process.