I hope this picture is visible.
I also tested it with 2,01 second cast, havn't uploaded that picture but it did not stack at that point.
Edit: I think the fact that scorch crit less than 1,5 second after fireball crit also produced the ignite stacking, that the threshold is just 2,0 seconds, anything faster than that will roll it.
Edit2: Another observation is that crits more than 2 seconds apart seems to delay the ignite tick, so that it
'stacks' like we've been used to, it's only under the 2 second interval that it seems to roll ala pre TBC.