In general, a lightmeter will not give you directly exposure times in 10's of minutes. Neither will it have provisions for the f-numbers of a pinhole camera (camer obscura). But that is the easy part: say, meter indicates 1/125 f/16.
"Equivalent" (read below) exposures: 1/2sec f/128, 1sec f/180, 2sec f/256, etc... but only a starting point.

The hard part is reciprocity law failure. And the meter won't help, even if it has markings up to 30min and f/256.