How long for a FW dip?

I've been looking now for 15 minutes through the clam forums and I cant find a solid answer. There is something wrong with my small Maxima that may be PM but isnt snails. I did a FW dip for just one minute and that already seems to have made a difference as the clam is starting to open a bit and extend its mantle but how long should a FW dip be? I've been told as little as 1 minute to as many as 30 minutes...how long should a FW dip be?
 
Everything I have read and from my own experience, 20 min. If the clam dies from the fw dip then it was pretty sick to begin with. Also keep in mind the clam stays shut and doesn't filter the fw, so basically the fw only comes in contact with the shell and byssal opening.
 
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