[williamcotton]

Transactions without fees will eventually be processed and normally within 24 hours in my experience.

On top of that miners are currently processing transactions with fees of 1000 satoshis with no delay. That's about $0.004 with the current exchange.

[hobo_mark]

Is this an artifact of current network dynamics or is it explicit in the protocol?

In other words, can I, today, send zero-fee transactions and reasonably (99.99% of the time or something) expect them to be processed by some _known_ deadline? (Be it 24 hours or a week or whatever)

[williamcotton]

It is just current network dynamics. Confirmation times in general are unknown as blocks are produced on an arbitrary timescale that the software attempts to trend to a 10 minute average by actively changing the difficulty of the proof-of-work mechanism to match the entire hash power of the network.

For transactions without fees there is no way to guarantee anything.