Ok, been looking at dewars... new seem to be only slighly more expensive than used, and the used usually have the caveat.. sold as is, no tested :(
so, for about $700 I can get a new dewar with a low evap rate.  Says it has a built in pouring device in the neck, but ???  They also sell a doohicky for only $500 that pumps the ln2 out.... hey, maybe I could just syphon it out 
 
Anyway, back to the old question, how long could I bench on 25 liters of ln2 considering I didn't spill any or do anything else stupid?  I'm thinking of the CPU and a Tech 5 in the middle of a 2 pcb 295.
Any ideas?