You may transplant it as soon as the plant is big enough to handle. I removed four 3/4" basal rosettes from my N. rajah when the mother rajah was only 2" in diameter itself. I used a sterile scalpel and forceps, my fingers are far too large and clumsy for such things. All offshoots were established in LFS and have grown to 3-4" in diameter. Basically if you remove a tiny basal rosette you will want to be able to remove a few of the lowest leaves so you have some sort of stem to plant to get roots to come out of. When the tiny rosette has 6-8 leaves you should be able to safely remove it and put it in a tiny seed pot of LFS to root.
My method was:
1) Set the pot at eye level (so both your hands are free)
2) Dip scalpel in alchohol and burn off with candle flame dip in water
3) Grasp basal shoot with forceps just above where you will make the cut
4) Slice off basal shoot and drop onto R/O water (you may add a few drops of Superthrive if you want to)
5) Dab Charcoal dust or fungicide (or even rootone with fungicide) on mother plant where basal shoot was removed to prevent any viruses. Not necessary (I've forgotten to do it from time to time) but why risk it?
6) prepare small pot with moist long fiber sphagnum
7) using forceps pick up basal shoot and cut of the lowest 2-3 leaves and using the scalpel make a couple slices down the stem where the leaves were removed (to expose the insides of the stem to air/growing medium- it seems to ease root development)
8 ) dip stem part of basal shoot into rootone with fungicide, shake off excess
9) wrap stem with long strand of long fiber sphagnum or a few pinches of living sphagnum if you have it
10) stuff into the small pot snugly so the plant doesn't wobble when you pick it up and set it down, if it wobbles it will not root as well.
Then just put it into the same conditions as the parent unless the parent is in low humidity you will need to keep a fresh basal shoot very humid, it's the same thing as a new cutting with no roots, mist heavily it daily but keep the soil barley moist until it shows growth.
Hope that helps some!