Time to sell.
I have done nothing but buy for the last 15 years. It has done quite well.
But if Wal-mart is truly mature and is no longer a growth business, it still has 5.5B in cash and can produce cashflow in the 20B range.
Right now the dividend yield is 1.25%. If they decided to just dividend more cash out to the shareholders it could be a stock yielding a reliable 6-7% dividend.
I'm not telling you what to do or recommending anything - make up your own mind.
But slowing growth is no reason to be a panic seller.