My army of fact checkers have been working on this feverishly for the last ten minutes and this is their conclusion:

It would take approximately 100,000 years for a spacecraft traveling at the speed of light to cross the diameter of the Milky Way galaxy, as the galaxy's diameter is roughly 100,000 light-years across.

Understanding the terms:

Light-year:

This is a unit of distance, not time. It is the distance that light travels in one year.

Speed of light:

This is the speed at which light travels in a vacuum, about 300,000 kilometers (or 186,000 miles) per second.

Calculation:

Since the Milky Way is about 100,000 light-years in diameter, and a light-year is the distance light travels in one year, it follows that traveling at the speed of light would take 100,000 years to cover this distance.

Key takeaway:

This illustrates the immense scale of our Milky Way galaxy, even when using light speed as the measure for a journey.