I usually read all the posts above mine to see if anyone else has made a similar comment that I wanted to make, and if so, copy their post, put it in a quote box, type "Agreed" and then click Submit.
If the above does not apply then I just type what I feel. Then I re-read it for proper spelling and grammar. And then Submit. Usually takes about 1-5 minutes, depending on how much I want to (or can) say.