I suggest you do some tests. The procedure is the same as for safelight tests. One way would be to lay out some paper with a few coins on it. Every 30-60 seconds, remove a coin, until they're all gone. Total time should be based on the time you expect a paper to be exposed to light, from its removal from its box to when processing is complete, but make the time long to be on the safe side. Once you've removed all the coins, process the paper. If you can make out the outlines of any of the coins, then fogging has occurred, and you can use the number of coin outlines you can see to estimate how long you can leave paper out in your darkroom. You might want to perform this test multiple times, for different papers and/or different locations in your darkroom (your enlarger and your developing area, say).