Eyes How ventilated is enough for working with Resin?

First thing to do is to find the Material Safety Data Sheet (MSDS or SDS) for the resin you use. Read it though carefully and see whet dangers are listed.
Then you go online or find someone to ask (preferably a professional, not a bunch of hobbyists on a forum) and figure out what , if any, protective gear you need.

Then you make sure to work in an appropriate place.
If the only danger is some solvent that dispurse quickly, it's possible that it's safe to use in your bedroom as long as you can open the window and get a good draft. Experiment with opening doors and windows in other parts of your home as well go get a good draft and make sure to keep it going until the resin is fully cured. And do keep in mind that some resins will keep giving off fumes for quite some time.
If, on the other hand, there are some more toxic stuff in the fumes you should probably find a better place to work.

Edit to add: A draft can cause problems with the curing of some resins, so keep an eye on that.

 

I highly recommend, even if you can get proper ventilation indoors or outdoors, to research respirators and purchase one with the proper cartridges. One thing about resin is that often times the kits will say they're non-toxic, but the mixed/uncured version actually is, so it's just best practice to gear-up each time you use it as habit, and ensure your lungs are protected longterm. I wear a respirator pretty much any time I work with something that smells (airbrush paints, primers, gesso, etc.) Furthermore, having a respirator handy means that if you plan to do any post-processing on your resin, you're already prepared to protect yourself from dust as well.

 

I like UV resin, but I certainly wear full PPE with a good respirator, and never inside my home as it has a single ventilation system. This works well for me since I work a night-shift, so it's not uncommon for me to be able to work at night in my garage, under a vent hood. I've yet to find a good UV resin that has a Material Safety Data Sheet to reference, so I figure it's better safe than sorry!

 

Remember, just for reference- this gets lost in translation and people's vocab: respirator means the heavy silicone mask thing with cartridges you must regularly replace on it, it doesn't mean dust mask with a little single exhale- valve and ear elastics.