Depends what I'm brewing but I use an ispindel and when I see the curve flattening out, which matches the fall of krausen or more difficulty in keeping the temperature up I let the hops in. If DDH then I put the second lot in when I have let temp fall to 14 celsius which is usually about 3 or 4 days after the first dry hop.
But I'm not sure if my method is optimal. If I go too early with the hops they smell wonderful throughout the house, but that's not where they should be!
If it's in a conical, when there's about 10 points left I drop the hops then bung it up to carbonate, if it's a bucket job then I leave it another couple of days. a lot depends on the OG and the temp, as to how long that actually is.