This is primarily for First Impact, though I think it applies to critiquing in general. I offer the monthly prizes to encourage critiques, but I don't want you to think you have to work hard for it.*
Your critique does not have to be long. When I say only "real" critiques are entered for the prize, I mean useful critiques. Saying "This rocks!" or "This sucks!" is not useful.
However: "This rocks! I love how clearly you lay out the protagonist's choice at the end" is useful, and those 15 words totally count towards the monthly prize.
You don't have to be an expert, just a reader. The point of first impact material is to compel someone to read on. Agents are readers just like you and me, compelled by the same things. All you have to do is say whether or not it worked for you.
You will learn by critiquing. I've talked about this before, but the more you critique something -- anything -- the better you will get not only at critiquing but also writing. The critiques are for you as much as anyone else.
All that to say: DON'T BE AFRAID TO LEAVE A QUICK CRITIQUE. Writing is subjective, so multiple quick critiques can actually be more useful to a writer than one person's (points at self) verbose opinion.
When in doubt, just say whether or not you liked it and a brief note of why. That's all you gotta do. Here, why don't you practice now.
* Although the long critiques are most certainly appreciated. ALL critiques are.