It's very hard to make a judgement. The most common time when I start to see a problem is the 'wall' of mist that appears maybe 3km in the distance going from the current cloud base all the way to the ground, and you can't see past it.
I slow down, and the wall rolls back as I progress towards it, drizzle hitting the canopy. I'm normally at over 1500ft above the ground, but I would progress in this situation as low as about 600ft. If the wall stopped rolling back, I wouldn't go into it.
Anyone got any rules of thumb about when to say stop? I think it's likely that the people who have accidents saw scenarios not unlike this prior to their incident - the degradation always seems slow, anyone got a magic trick which can help make the decision?
BW