Question:  How long does it typically take for the USD exchange rate of bitcoin to double?

Answer:

Start_rate	End_rate	#_of_days	Dates
$3.125	$6.25	194	2011-12-01 to 2012-06-15
$6.25	$12.5	163	2012-06-15 to 2012-11-25
$12.5	$25	79	2012-11-25 to 2013-02-12
$25	$50	30	2013-02-12 to 2013-03-18
$50	$100	14	2013-03-18 to 2013-04-01
$100	$200	8	2013-04-01 to 2013-04-09

Given this timetable, would anybody like to make a guess at a range of dates between which they believe the exchange rate will reach $400?