Recip failure on long exposures

Greetings All,

I'm new here so if this has been discussed before, my
apologies.

I am a cinematographer and got interested in pinhole while
shooting commercials -- where for obvious reasons, very long
exposures aren't an issue. Now that I've got a pinhole still
camera, however, I am interested in scenes that call for 7,
15, even 30 minute exposures before adjusting for recip.
failure. Problem is, as far as I can tell, neither Kodak or
Fuji offer any guidance on exposures of this length. If
anyone's got any info or experience to share, I'd really
appreciate it.

Cheers,
Stephen M. Harris