I raise you a 115km bus route running on a 15 minute frequency, the 901 in Melbourne.

This route might not be as straight as the other two recently posted here, but it's 115km long and takes four hours to complete the entire journey in one direction.

Though in this case, it's not intended to be taken from end to end, but provides an overlapping feeder service between (checks on a map) seven different train lines.

There are two other routes that follow a similar pattern, forming three concentric rings. The Suburban Rail Loop project aims to do something similar yet totally different with a fully automated underground heavy rail service with very widely spaced stations.

With all that said, Melbourne's buses are often forgotten about and they definitely don't get any interesting upgrades, so these services aren't as effective as they could be and don't meet the standard of BRT.

It's worth noting that particularly with this being the outermost orbital bus route, this passes through mostly very low density suburbia.