I think mine usually start to sprout true leaves within a week of putting out the cotyledon leaves.

Thanks, seil!!!

I tried embryo extraction for the first time, practicing on OP hips that had been exposed to a week of 0-10 degrees F. I didn't expect any of the seeds to still be alive, but was surprised with two viable embryos. Sadly I damaged them in my extraction attempt, but their cotyledons have greened up and they are now growing roots. And the bigger one already has what looks like could be the start of its true leaves, although I'm not sure -- it looks a little odd. I guess I'll find out for sure soon enough. :)