Early conversion, I tried 5 hours a day with an XM20K and a CV20K side by side, they put quite a hurtin' on some of my critters. I would infer I should have raised the lights higher and possible reduced the exposure even further.
My VHO setup was on IC660 driving 2-daylight and 2-actinic white bulbs.
Lately I have been using a separate reflector and burning in over curing rock instead of over the tanks.
fwiw: Going from the VHO's to IceCap 10Ks on another tank was fine with the 5 hours.
fwiw2: currently running a 48" with XDE20K-IC10K-CV20K with the the 20K's on for 12 hours and the 10K on for the middle 6 hours. Consider this a long term test ;>)
Not a great response, but I hope it helps.