I think the best answer here is it varies ball to ball.  

I drilled two of the same ball.  I saw a drastic reduction in performance in one of the balls.  I gave it a hot water treatment, resurfaced it, and tried everything to get the reaction back.  It just never came back.  However, the other ball was fine and never lost much reaction at all.
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RIP Sawbones

Both of the above are correct.

When I was throwing Hammer, I had a SledgeHammer that was on its sixth drill I believe....and two of those drills were mine.  I had many...many...many games on it.  Ball just seemed to roll like no one's business and I had strong reaction all the time.

More recently, gave all my Visionary stuff the bath treatment and they all came back to life....some too much....that I had to get a little polish on the ball to get more length of out the ball since now it's too strong.

Life also depends on the coverstock though.  Some will start to crack as a result of lack of thickness and weather, depending on where they are stored.

I would use until you feel there's nothing left.  Then start looking for the next best thing!!
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BR.com.....going down the toilet one nugget at a time!!  

g thing is back....with a vengeance!!  

www.visionarybowling.com - Accept no substitute for the very best there is!!