Urgent - Track Assignee Labor Time
If you want to track how long a ticket was assigned to each assignee (not the amount of time they worked on it, but how long it was assigned against a clock--24/7, day shift, etc), you can use clocks.
If you want to track actual time worked, you could tailor something to allow them to enter the actual work time. You may be able to leverage the parts & labor table section from the OOB forms (it is normally hidden, but can be exposed).
However, if you need to be able to report on labor across modules (time spent on IMs, PMs, KEs, Changes, change tasks, etc) then using fields based on each record can greatly complicate reporting. If that's the requirement, I would suggest tailoring a solution which allows a user to enter work records against a ticket but tracks them in a single file (sort of like activity records if the same activity table was used for all modules).
You can use the OOB functionality to get the total time spent by the assignee on the particular ticket.
Go to the DB-> sloreponse table check the field named "Elapsed time".