How long is the commute from Croydon to CBD during the work week?

During peak hour I'd allow hour and 15 minutes so you're not late - if it's not peak hour only about 30 minutes depending upon where you are going in the city too. The train is often quicker and cheaper due to parking charges in the city which can be quite expensive - especially if you don't get the early bird special prices (in by 9am out after 3pm I think)

It depends on your expected ETA. If you want to arrive at 9am I would suggest 1-1.25 hrs.
8am - 1hr
7am - 0.75-1hr
On an express (most are during peak), train timetable suggests 37mins to Parliament.

Give yourself a good hour. Unfortunately the eastern fwy banks up from Springvale to Bulleen rd every morning

The train is 40mins Croydon station to Parliament.

Wife does the drive from Hull Rd to Abbotsford in 45-55mins