I would say 6 months to 1 year for fluorescents such as 805. The fugitive dyes will have a tendency to diminish with time.
UV curable inks would be 1 year.
Regular oil based inks can be 5 years. Be more careful with blacks as the carbon containing ink can absorb the driers and effect drying.
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Tried and true method as follows: 8.5 x 11" coated solid sheet will use 30 lbs. for 100,000 impressions That's 93.5 square inches. 8.5 x 11' uncoated solid sheet will use 45 lbs., again 100,000 impressions. You then can calculate any job, coverage based on total square inches and print run...
Ask your ink supplier for a non-yellowing overprint varnish. These products are water white and should work great at sealing the sheet for your dry trap color to POP!
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