"20000lumens"

I doubt it.

"Anyone know about how many days it takes for sunburn to show up?"

UV damage is apparent within a couple of hours of exposure to the sun, same as for people.

I meant 20k total.

1000bulbs.com/product/88858/SUN-960300.html

So, anyone think 12 inches to start will burn anything?

It won't "burn" anything, even up to 1 inch, but it can slowly kill some shade loving plants such as African violets.

12" away to start is a pretty safe bet. When in doubt, put the plants near the end of the tubes or edges to start. Don't run the lights on for the full day in the beginning.

You must watch your plants very carefully for the first couple days for signs of stress and burning (changes in colour, curling of leaves, white spots, etc). From there on out, increase light duration and decrease distance to the lights.

When receiving new plants grown in greenhouses, I typically put them 8" away from my T5HO lights to start. The bulbs in my lower light setup are mounted 5" apart, center-to-center.

If the plant response is favourable, each week after that I move them closer to their final desired height and location (which could be under my other/brighter grow light setup).

Thanks ewwmayo, that's some good advice :)