Depends how fast you accumulate them. I like to keep at least 2000 on hand.

I get 50-75 in an average day.  That may be a little lower for the next week.  I need to farm dragon20 instead of brutal 12-3.  I should have 2000 by next CvC

Haven't gone below 6k in a long time and would really not want to. Never know when you're going to need to develop a good case of carpal tunnel and finish off frags in a Summon Rush or Champ Chase. 

i dont try for frags.....i never complete one.....i have multiple champs with 0-60 frags...never completed

You will, eventually. :) As I said in another thread, it's a matter of getting your resources right, through teams in the right content and saving up for fusions. 

This ^^^

You should be doing all the events during a fragment event instead of the "off cycle " events. Still use all your energy to progress (make chickens, masteries, whatever...) but no need to push during downtime until you are able to do fusions as well.

When I can farm all the dungeons at 20, I will try the frags.  

Do frag events offer more value than CvC?

Frag events (fusions) give you a Legendary. This is an easy Yes.

I wont be doing 1,000 or more mystery shards again soon....i overloaded myself with rare champs......i ran out of silver getting the shards out.........and then ranking them up for fast food.....learn from my mistake.....be prepared.....what should have been a few hours....turned into 13 hours of grinding work.....not fun

This what was I was warning you about the last time you asked. It's a major time drag :)

Once I could only get 10 shards at a time, instead of the usual 40-50 it became a major time issue.  When I did 800 before it was only a little over an hour.