I'd guess you'll have your 50 GH/s miner within 1.5 months (around mid-November) at the rate BFL has been recently been moving along with their order queue, maybe even sooner.  

If they keep up with their rate of about two weeks of preorders per business day (including Saturday), all July 2013 miners will be shipped by around the 16th of October.  Giving them 1 month of orders per 3 business days, that puts all July orders shipped around October 21st.  Assuming there was probably a large spike in orders around April 2013's huge increase in USD/BTC, I'd still estimate them having July orders shipped by the beginning to middle of November.  At this point they've gotten production down to a science and now they're just cranking them out full speed. (although they seem to still be having issues getting the fans connected correctly, at least on 2 of my units)

Based on BFL's history their estimated delivery time of "February/March/whatever they say" for the Monarch may end up being a lot later than that.  Of course they may have also learned a lot from their current round of products and may now be giving a realistic estimate, but you have an almost guaranteed chance of getting the 50 GH/s miner within two months and your delivery of a Monarch is a big question mark at best.  Oh and the 10% transfer fee. (are they still doing that?)

Mining dashboard for 50 GH/s miner, start mining in November 2013: http://mining.thegenesisblock.com/a/3ee7c772cf
Mining dashboard for 600 GH/s Monarch, start mining in April 2014: http://mining.thegenesisblock.com/a/eb4131e27c
Both assuming 40% difficulty increase per month. (that might be a bit conservative, but even with that neither look very good. :/ )