I personally don't think it matters at all with any site but Shutterstock.  I try to upload one batch a week on late thursday night or friday morning.  That way they are usually approved and into the search results by monday morning when things start rolling again.

For best time to upload is when I sleep so I fire the script and go to bed :-)

Or when I go to work

with approval times so varied when you upload has little relevance. i suspect most folks would like new pics to show up during the week and not the weekend that can be hard to judge.

Fridays. No more than 10/20 at a time.

For Shutterstock: friday, mixed batch of 10-15 max. Don't ever upload series of the same shoot to SS: they all get the large exposure at once and they will compete with each other. I make mixed batches of nature, concept, business and lifestyle models. That way, you'll drag a wider customer base into your existing portfolio.

For the others: time doesn't matter but keep the batches 10-15 and very mixed. If Atilla strikes, not a full shoot is lost.

Good advice FD.

donding, I am not sure , after reading the answers here, if time and days make a big difference , as my days are the opposite of the others. I thought about this earlier last month as well, eg. am I getting more approvals by uploading at the end of the week, or the other days.
Strangely enough, there seems to be a correlation between my approvals and the time and days I upload to certain sites. But this could just be coincidental.
However, I sure wish there is a science to this, as I could use it to my advantage, to get more approvals and get my images to be reviewed by the reviewers who seem to like my images.
Then again, I think it's just that my images are so much better nowadays