This is more a rule of thumb for the receiver; if you send a transaction (based on coins which are already well-confirmed), and don't try to send any conflicting transactions, then it shouldn't disappear as long as it gets one confirmation (even if the block chain forks, it will be resubmitted)
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Also - always wait for your wallet to show at least five (5) confirmations before being (reasonably) certain that your transaction has committed.
General rule of thumb is it takes about 60-90 minutes to be guaranteed that your bitcoins have transferred.
This is more a rule of thumb for the receiver; if you send a transaction (based on coins which are already well-confirmed), and don't try to send any conflicting transactions, then it shouldn't disappear as long as it gets one confirmation (even if the block chain forks, it will be resubmitted)