Collage elements appearing one-by-one
There's not an industry standard, but two seconds is too long for logos.  If it were a sentence of text, then maybe, depending on how long of a sentence.  

A colleague of mine once told me that you should keep text on screen for long enough for you to read aloud, backwards, one word at a time.  So, if you put, "The quick brown fox jumped over the two lazy dogs" on screen, you'd read: "dogs-lazy-two-the-over-jumped-fox-brown-quick-the" aloud, and it should take about as long as the text is on screen.  I can do that in under two seconds.  

Logos are specifically designed to be identifiable by a quick glance, plus, you're talking about popping up a tile, and leaving it up, correct?  If you weren't, and each logo disappeared before the next one came up, I'd say 15 frames might be too long.  If each logo is readable after the next one pops up, you might not pause at all, depending on how they're animated, how simple/complex they are, how busy your other layout is, etc -- but I'd experiment with between 1-5 frames.  

People's attention spans are shorter and shorter every day.  Cuts in movies and online videos are faster and faster every day.  Your audience is very good at identifying new information quickly.  Don't sell them short.  They'll get bored and watch something else.

Also, when a client asks for a video or video element of length X, the first revision you show them should be X/Y units of time, where Y is a positive integer, which increases the less your client knows about video.  For the average person, I'd put Y at about 3.  So, the first version I show a random person should be about 1/3 the length they'd requested (It's probably smart to let them know up front, though.)  In cases where the time is fixed, like music videos, or commercials, substitute time-of-effort for video duration.