The Jargon File (version 4.4.7, 29 Dec 2003):

tube time
 n.

    Time spent at a terminal or console. More inclusive than hacking time;
    commonly used in discussions of what parts of one's environment one uses
    most heavily. ?I find I'm spending too much of my tube time reading mail
    since I started this revision.?


The Free On-line Dictionary of Computing (30 December 2018):

tube time

   Time spent at a terminal or console.  More inclusive than
   hacking time; commonly used in discussions of what parts of
   one's environment one uses most heavily.  "I find I'm spending
   too much of my tube time reading mail since I started this
   revision."

   [Jargon File]