In Chapter 617, the Mimihagi reached the Soul King in Soul King Palace from Seireitei's Shakonmaku in just 2 pages
http://a.mangatown.com/store/manga/9/58-617.0/compressed/n007.jpg?v=51425267067
http://a.mangatown.com/store/manga/9/58-617.0/compressed/n008.jpg?v=51425267067
http://a.mangatown.com/store/manga/9/58-617.0/compressed/n009.jpg?v=51425267067
http://a.mangatown.com/store/manga/9/58-617.0/compressed/n010.jpg?v=51425267067
Meaning this should be faster than the Vandenreich feat:
http://a.mangatown.com/store/manga/9/58-627.0/compressed/h008-009.jpg?v=51432258026
http://a.mangatown.com/store/manga/9/58-627.0/compressed/h010.jpg?v=51432258026
,http://a.mangatown.com/store/manga/9/58-627.0/compressed/h011.jpg?v=51432258026
http://a.mangatown.com/store/manga/9/58-627.0/compressed/h012.jpg?v=51432258026
http://a.mangatown.com/store/manga/9/58-627.0/compressed/h013.jpg?v=51432258026
http://a.mangatown.com/store/manga/9/58-627.0/compressed/h014-015.jpg?v=51432258026
As per suggestion, it is only logical that this should be somewhere around 5 minutes at a maximum.
5968589760/300=19895299.2 M/s, Mach 58003.8, or Sub-Relativistic+. (0.0664c)
This is a "minimum", he should at least be a higher Sub-Relativistic+.