When I leave a developed, stopped but not fixed piece of paper exposed to light (e.g. coarse test strip), it turns reddish after some time. My conclusion is: a effect that becomes visible in 15 mins starts developing immediately when the light was turned on. During 1- 2 mins it can be neglibile, but I prefer to be on the safe side. In other words, I'm with Ollie.(posted 8170 days ago)