For the sake of clarity, I rounded the times up to either the "0" or "5" mark - since I probably had a margin of error of a few seconds here or there. I counted whenever the character was on screen (in robot or vehicle mode). In cases of hectic, shaky cam action (for example, Prime fighting the Fallen at the end) I just let the chronometer run until it was clear the character was off-screen (or dead, as the case may be).
Here they are, in order (with some screenshots to make it look cool):

The Twins: 6 min. 40 sec.

Jetfire: 6 min. 30 sec.

The Fallen: 5 min. 10 sec.

Devastator: 3 min. 35 sec.

Wheelie : 3 min. 10 sec.

Sideswipe: 2 min. 15 sec.

Rampage: 1 min. 45 sec.

The Doctor/Scalpel: 1 min. 40 sec.

Soundwave/Dr. Claw: 1 min. 35 sec.

Ravage: 1 min. 35 sec.

Ratchet: 1 min. 35 sec.

Demolishor: 1 min. 15 sec.

Arcee(s): 1 min.

Long Haul: 45 sec.

Mix Master: 45 sec.

Grindor/Blackout: 45 sec.

Sideways: 40 sec.

Jolt: 35 sec.
I think this is a good indicator of how fast this movie moves at times. When you actually count the seconds go by, you see how little time there is to take everything in.
Next time, I should count how much time Bay spends focusing on military equipment and soldiers running around... I'm sure it's a solid 10 minutes of the movie...
Hope this has been interesting for some...