When I print at such huge sizes, the print guys usually request 100dpi.
This is to avoid having to deal with huge files and also because often the banners and such are not designed to be looked so close as as a magazine.
Take a close look at posters in window shops, you'd be surprised how pixelated they often are.
Maybe in this case this is more an art print so best practice would be to check first with your print guy for his advice because no need to provide a huge file if he lowers the resolution when he receives it.