Here, we see Vegeta shoot off his Final Flash and it extends past the planet and explodes after travelling a distance deep into space, and then it explodes. Ki attacks typically don't explode unless they hit something, and the Final Flash has never shown to explode without hitting something, so it presumably hit a celestial body. One could argue that it was one of the planets in the solar system, but if one of them were destroyed, it would have been mentioned in the Daizenshuu 7 or the Chousenshuu 4 Geographical dictionaries where they talk about the planets in our solar system, and they don't, so it was presumably travelling to and destroying a planet or a star at an interstellar distance.
For this calc, I'll be using the distance from Earth to Proxima Centauri, which is roughly 4.22 light-years. The blast travels the distance in 27 seconds, which would be 0.15629629629 light-years per second, or 4932335.999801304 c, which is Massively FTL+, and Perfect Cell was able to somewhat react to it.