I'm thinking of trying my hand at extremely long exposures, in the order of days, weeks, months and maybe even years. The plan is to exploit the reciprocity failure of some films to get the long times I'm looking for, especially Kodak 400TX since it's the worst of the of "standard" films. My question is this, what to do with varying light conditions. For example, as a test, I intend to shoot a shot of my apartment for couple days. For maybe 12 hours per day, the apartment will be lit by sunlight/artificial light, and during the night it will be dark. In my head, I'd figure that if I meter for the light part of the day, and then overexpose one stop, it should make up for the darkness half of the time. Is this right, close to the truth, or completely off? And how accurate must I be when doing these exposures? I mean, if I expose a piece of film for say a month, how much does it alter the exposure if I cut the time short by a couple days, or forget about it and stop the exposure a week too late?