How long will it take for OpenCoin to distribute the 50 billion XRPs? Out of the 100 billion XRP that can ever exist, the founders kept 20 billion while OpenCoin received the remainder of 80 billion. From that 80 billion OpenCoin will sell 30 billion to fund ongoing development beyond the initial angel investment, and the remaining 50 billion will be given away to spur adoption.

How long will it take OpenCoin to distribute the remaining 50 billion? One year from now, how many XRP will OpenCoin give away to each unique new user? What is the smallest amount that OpenCoin will eventually give away, over time? If there is no specific answer to the time, can OpenCoin provide a range (for example, between 5 and 50 years)? Is there a reasonably estimable lower limit on the time required to give away the 50 billion? Is there an upper limit on how much will be handed out (i.e. will anyone ever receive 50,000 XRP again as part of the giveaway)?

Answers, even if partial, appreciated.
The short answer is that nobody knows. In the short term, the rate of XRP release will be used to manage the network's growth. Once it transitions to a distributed network, that won't be an issue any more and it will likely then be limited just by how quickly we can give them away.