Correct me if I am wrong, but I thing waiting 48-72 hours is sometimes not necessary.
If you send transaction with very low or no fees it will not propagate in the network very well. It might even get stuck after being announced to a few nodes. If you send coins again with the fee and
use only the nodes that has not received your first low-fee transaction then you do not have to wait more than several minutes. Using only uninformed nodes should not be difficult as this will be majority of network (your low-fee transaction is stuck only on several nodes.)
I think you are right but i really want to know how to use nodes that has not received your first low-fee transaction because i sent 0.01btc from blockcain.info with 0 fee and it wasn't refunding those coins after 24hours so i decided to double-spend on my bitcoin qt 0.8.6 while it was synchronizing(about 20hours behind or so) and no active conections to the network i sent those coins and then i checked the tranasctions id on blockchain.info/tx and other blockchain explorers it sayed that tranasction dose not exist what could be wrong and why and why does this happens?