This depends on equipment type and starting conditions:

50L Dewar at -196°C:

Day 1-30:    Samples perfectly viable
             Temperature: -195°C to -196°C
             
Day 31-60:   Still excellent condition
             Temperature: -188°C to -195°C
             
Day 61-90:   Good condition, but monitor closely
             Temperature: -180°C to -190°C
             
Day 90+:     Risk increases significantly
             Temperature: Below -180°C
             
CRITICAL: Never exceed 90 days without refill

 

20L Dewar at -196°C:

Duration: 25-35 days before hitting -150°C threshold
(Shorter lifespan due to higher evaporation rate)

 

100L+ Dewar at -196°C:

Duration: 60-90 days minimum before concerns
(Better insulation = longer stability)

Best practice: Never run dewar below 10% capacity (2-5L remaining). Always refill when 20% empty.