Enough that a missed delivery is an inconvenience rather than an incident, and the figure comes from your own consumption rather than a rule of thumb.
Establish a baseline in litres per week in your own room, at your own workload, once the vessel has settled after installation. Then set an action level, the point at which somebody orders rather than notices, and write it down before you need it.
That action level should be higher for vapour phase storage than for liquid phase. In liquid phase, a falling level still leaves samples immersed. In vapour phase the samples already sit above the liquid, so every centimetre lost pushes the warm end of the temperature gradient further down the canister.
Plan the reserve around the longest realistic gap, which is usually a bank holiday weekend rather than a normal week.
