Cloners?

I am using a cloner for the first time. I have several strains going but am noticing several of the cloned strains are showing solid roots forming. On the other hand 2 of the 4 strains are not showing roots yet. I know every strain is different and therefore the may just take longer to root. Does this sound right?

Yes that's absolutely right. Some strains take much longer to root and some strains just plain don't want to root at all. I had this problem with Grandaddy haze. Couldn't get roots out of it no matter how hard I tried or what method. Best of luck and happy growing.

At what point do I transplant and put in soil (how long should the roots be at minimum)?

as soon as the roots show you can transplant into first pots, however if there all going into the same space for your next grow i would wait untill theyve all shown b4 potting on, just so they all get the same start, makes life easier,

normally once the first signs of roots show, the others are not far behind, probs in the next day or so they will all be showing.

I personally like to see at least 1/4 inch roots before transplanting.