It would depend on a lot of things, such as terrain, weather, traffic etc. If you are not sure, simply ride a few kilometers and you would be in a position to guess. Safety hazards increase as you get tired, it will affect your judgment and reaction speed. You can get a guess on that by doing some trial riding.

One thing is for sure is that the more you ride, the more safer you will feel. Your body will adapt to cycling and you will feel less tired day-by-day, and be more confident about yourself on the streets.