Working with liquid nitrogen all day, I am constantly bothered by their boiling away, forcing me to go get more from the storage dewar. Likewise should be the case with any cryogen, e.g. LH2 and LOX that are the most common rocket propellants.
From what I've read, nearly all robotic probes use propellants that don't require any refrigeration: hydrazine, MMH, N2O2, and so on, as they can sit happily in a steel ball for years. Driven by another question that brings up the proposition to "bring hydrogen to Mars" to produce rocket fuel, my query is how practical is that? Hydrogen is not liquid at any (reasonable) pressure unless cooled below ~33 K, a difficult feat to maintain that requires lots of insulation and/or evaporative cooling.
What is the longest that any mission (manned or unmanned) has stored cryogens? (used for fuel; WISE doesn't count). Apollo at a couple weeks? I can't find what Soyuz/Progress use, or the ISS's engine, the Shuttle's OMS seems to use MMH/N2O2.