From Robert Openshaw, 02.09.17:

From 0% to 100% helium seems to take ~ 80 minutes, from 100% helium to
0% helium takes ~ 120 minutes. The reason the decrease in helium takes
longer is that the max flow of CO2 is 70 cc/min, while the max flow of
helium is 100 cc/min. Note that for "small" changes you probably don't
have to wait as long, e.g. from 45% to 50 % He if you only wait until
mix is 80% completed you still have ~ 49% helium (which is probably
within error of MFC's).


The data that was based on is here, with chart:

  http://midtwist.triumf.ca:8081/EL/020726.5488?exp=twist