Memo from Boeng
My company sent out a memo from boeing which I don't have to hand so can't get the exact figueres but stated that;
With gear retraction (system A fluid is now in the actuators) a fall of aprox "xxx" % can be expected.
Under certain conditions, especially long cruise below ISA (to be expected this time of year) thermal contraction of the fluid will result in a further decrease of up to "xxx" %.
Sorry no exact figures but I do recall that the quantity can get very low without being a non normal condition, however QTY's should obviously be checked after landing.
Hope this helps, perhaps someone out there has the same memo to hand to share the figures.