The following code should do it:
WAApplication allInstances do: [ :application |
application keysAndHandlersDo: [ :key :session |
| policy table |
policy := application cache expiryPolicy.
table := policy instVarNamed: 'lastAccessTable'.
Transcript
show: session; show: ' expires in ';
show: policy timeout - (Time totalSeconds - (table at: key));
show: ' seconds'; cr ] ]
Note that the above code accesses internal data structures that might change in the future. Also you might need to add additional checks to make it work with your setup.
Also note that you might get negative seconds. This means that the session is supposed to disappear, but it hasn't been reaped yet.