So best case scenario is that it brings you from 64/480 (no frozen targets at all) to 100/480 (all targets frozen all the time). I suppose we could assume you swap some CHC rolls for AD/CDR/damage range, but let’s start with a basic crit build and see where that gets us.
(1 + 1.00*4.8) / (1 + 0.64*4.8) = 1.424, i.e. you gain a 42% DPS increase from the guaranteed crit against a frozen target if you have baseline 64% CHC.
Then there’s the value of an extra 20% cold on top of that. Going from 40% to 60% cold is a 14.3% dps increase, or 13.3% if you use elemental on Enchantress (so going from 50% to 70% cold).
Together, that would make Frostburns worth as much as 61-63% dps. A little less I suppose since you still get from freezes from Buriza itself, but in my brief testing Frostburns seemed to give freezes a lot more consistently than the innate Freeze from Buriza.
This is, of course, assuming it gives you full freeze uptime with Frostburns. The value will decrease the more you build CC immunity on mobs. I expect it’s worth more on average in your 5-7 min runs (where you can kill some enemies, especially trash, before it hits CC immunity) than it would be in a max tier push. I think once you hit CC immunity you can probably only expect 30-40% uptime on freezes after that if you keep attacking the same targets. At which point, it’s definitely worth less than CoE.
But in mid-tier runs, there’s definitely an argument for it over CoE. Before you hit CC immunity, it has the potential to be better than CoE assuming high enough freeze uptime.
In higher pushes you could also try and increase the value by bouncing between packs. That’s the old Vyr’s trick for Arlyse + APD builds when they first added CC immunity. Bounce between two different packs every 5-10 seconds, so just when one pack is getting CC immune you go back to the other, which in the meantime lost a good chunk of it’s CC immunity buildup. It’s not perfect, but it’s fairly effective. If you have one really big pull you can also just move side to side in the pull because you won’t be freezing distant targets on the far side very often in that scenario.
The same principle should work nicely with GoD + Frostburns, if you can make two smaller packs and bounce between them you’ll get more value from Frostburns and should end up with higher DPS on average. Try to be aware and play around CC immunity to increase the value you get.
EDIT: I’ll point out one place it will basically always be worth less than CoE though: RG fights. RGs (and yellow elites) have stronger CC immunity than trash mobs, and in a long boss fight you’ll spend a fairly long time after CC immunity has started kicking in. So vs. the RG, you’ll both spend a higher proportion of time fighting after CC immunity has started kicking in, and the CC immunity the RG has is stronger, so you’ll get much less value from Frostburns vs. the RG. CoE will be significantly better vs. the RG in basically anything except speed rifts (basically if the RG kill takes more than 1 CoE cycle, CoE is probably better).
EDIT 2: There’s also some value in the defensive utility of the freezes. Which should also help a bit with Squirt’s uptime.