Quite a while actually.. Unlike the V2/A4 which used liquid oxygen and alcohol the Wasserfall used Butyl Ether and Red Fuming Nitric acid as fuels, which were well on the way to being pre-packed... this is very close to what was eventually used in Lance... they were also hypergolic, which was nice!
RFNA did have problems though, as it would eventually break down and emit nitrous oxide. This was eventually solved by the Americans who developed Inhibited Red Fuming Nitric Acid or IRFNA.
John Clark's book covers RFNA and IFRNA in as much detail as you might want. The detail that sticks in my mind is the railway tank cars that sometimes burst, poisoning everyone nearby, because of the combination of corrosion and nitrous oxide gas build-up. Also the US built a fleet of special railway wagons lined with (I think) a special grade of stainless steel, at huge expense, and then scrapped them after IFRNA was invented.