different adcoms have said different things. obviously it first depends on the activity. personal anecdote on this experience is as goes:
I wrote really really short toolish phrases for description last year. Like for my TA experience I wrote: was a TA for X Y Z classes, thats it. I thought all applicants were the same and they would make correct assumptions and that I would atleast have a chance to explain in interviews. When I got to the interview stage, my interviewers seemed more curious about my activities. So after a couple of interviews, I wrote long-detailed description of EACH of my activities as update letters and sent them to schools. Now I'm sure some schools don't like update letters or reading long notes, however I got a few interviews after that.
When I got to one of my interviews, the person was also in the ADCOM. He told me that I didn't write much at all for my ECs and that was primarily what they read. He then told me that my update letter was what got me an interview.
Sooo.. I guess if I HAD to do it, I would write better, passionate, and exciting descriptions. Nothing too long. Not many experiences deserve all of the 1350 character limit, but if you cured cancer, go for it. However, I would not hit the Awards tab, write down Merit Scholarship, and do a play by play of my PSAT exam experience.
Treat it like an exam that you can review everyday until Jun whatever to make it perfect and effective.