Hi
Hello,
Yes, you will still need a TRV to re-enter the country from the US even though you have a work permit. The work permit only allows you to legally work in Canada. It does not give you the authority to enter the country.
I'm not sure how long it takes. You might want to call CIC or visit your local CIC office and ask them just to be safe.
Wrong, the OP doesn't require a new TRV if his/her intention is to only visit the Continental US or ST. Pierre et Miquelon, s/he will be readmitted to Canada as long as the work permit is valid.