Digital BW, The Print

Hello, 

Would anybody be able to tell me about the longevity of Epson K3 inks
(once opened),  and perhaps some rough notion of average number of 
prints per cartridge.  I  am considering purchasing a 3800 but have 
some doubts about being able to generate enough work to make it cost 
effective. Also is there any way of temporarily sealing the cartridges 
for long periods of inactivity?  

Thank you 

Eugene Appert