Q.What does "Good Time" in Colreg Rule 8 "Action to Avoid Collision" paragraph c means?

Good time is the time of Risk of Collision first began to apply.

The terms "Good time" and "Ample time" in my opinion, are used within the COLREG's to allow officers and masters room to apply good seamanship. It would be very difficult to define a term to suit all vessel. For a large cargo ship the OOW of the give way vessel my choose to make an alteration at 12 miles to maintain a set minimum CPA required by the masters standing orders. Other coastal operating small vessel might consider an alteration a 1 or 2 miles perfectly expectable.

To give you more of a solid answer how about this:

Good Time = As soon as it is apparent that a risk of collision exists so long as the proposed manoeuvre is safe to execute and will fulfil the requirements of rule 8.

in advance,

Good time means sufficient time in which close quarter situation could not be developed by the action of 1 vessel 

A good time or within ample time means the situation is understood well before whether vsl will pass safely or you need master 

Early