will broody pass on it's own or do we have to break her of the habit every time?
Broodiness will eventually pass on its own. But I've had hens sit for 6-7 weeks before they quit on their own. I had one hen in the past that spent pretty much all summer broody: sit for 3-7 weeks, come off the nest for a few weeks and lay some eggs, start sitting again... That particular hen laid well all winter when others were taking time off, so she did produce a reasonable number of eggs in each year. I wish I'd been in a position to hatch chicks from her!
I have had a number of hens go broody at various times when I didn't want chicks, and I haven't had any die of it yet--but I know that some people have had broody hens die of starvation, or of just being weak enough that something else (parasites, disease) killed them.
I wish there was an easy way to measure how much fat a hen has: because some backyard hens are prone to getting fat, while broody hens tend to lose weight, and it would be so nice to know which hens are going too far which way!
We have an isolation cage but just wondering if there is a better way ?
Isolation cage probaby is best because it's fastest--over in just a few days. I feel that putting her in a cage with food, water, and maybe a perch (but no bedding or nest) is less cruel than some other methods (like dunking in cold water) and easier than putting cold things in her nest (and replacing them when they get warm.)
If you don't want to do that, you might consider taking her off the nest once or twice a day and putting her near the feeder--that might encourage her to eat, and thus help her to stay healthy. (But it won't stop her being broody.)