An answer I posted in a noob thread, what I gathered from the Wiki, how many confirmations required before resending coins is TX-fee-free:

Transactions need to have a priority above 57,600,000 to avoid the enforced fee in the official client. Transaction priority is calculated as a value-weighted sum of input age, divided by transaction size in bytes:

priority = sum(input_value_in_base_units * confirmations)/size_in_bytes

For sending 0.05 BTC (5,000,000 base units) from one address, in 225 bytes (a minimal size, a one input, one output transaction with no remainder)

free when confirmations > 57,600,000 * 225 / 5,000,000 -- 2,592 confirmations

Having Bitcoins from multiple payments fund the transaction is where the "sum" in the formula above comes into play.

A more informative dialog in the Bitcoin client could be "This transfer requires a minimum .0005 bitcoin fee due to recently receiving the source funds, and the size of the transaction in bytes. This transfer would be free after xxx more confirmations (approximately xx days)"