The truth is that nobody knows. Only a few studies have analyzed the block chain to link addresses, and those were mostly big, important addresses like Mt Gox (see the work of Adi Shamir). I have seen plenty of people on this forum say that they buy bitcoins and transfer to a few ewallets, then to their SR account, and they are still around. Apparently, LE isn't savvy enough to analyze those links yet. That being said, those transactions are in the block chain forever, so somebody could link your transaction history 10 years from now. The more mixing you do, the better. My recommendation is to buy 20% more coins than you need and send them through multiple mixing services. One or two of them may be run by LE, but it is incredibly unlikely that all of them are, so the more the better. Simple transfers between ewallets do nothing to enhance your privacy. The coins are tainted 100% by the previous transactions, which form a direct link back to wherever you bought them.