How does LC50 translate to "real world" exposure for paint thinner? Last year I started oil painting, which means (in part) that I use paint thinner for working with and/or cleaning brushes. I recently became concerned about my exposure to the vapor from the paint thinner I use, so I requested a toxicology report from the manufacturer. In the document it specifies that the summated $\mathrm{LC}_{50}$ for the product is 21400 mg$\cdot$ m$^{-3}$.

My (basic) understanding of this is that 50% of test subjects will die with a single exposure at the above level; however, I've really no idea how that translates to "real life" and wonder if someone can shed some light on it.

I'm painting in a reasonably small room (around 3 m x 5 m) with windows open, so I'm essentially trying to determine if this is safe or not, or whether I should be using breathing apparatus and/or move to a different room.

This is a link to the whole document which I received from the manufacturer which contains the substance and other information.
The $\mathrm{LD_{50}}$ and $\mathrm{LC_{50}}$ (lethal dose and lethal concentration) values are measures for acute toxicity as you noted. Long-term exposure is generally not depicted well by these acute toxicity measures. A.K. pointed out, how improbable it is to have a serious risk of acute poisoning. (Also note that the vapour pressure is given as $2~\mathrm{mmHg}$, which is $2.7~\mathrm{mbar}$ in proper units, meaning that evaporation will be very slow at ambient temperature and pressure.)

Chronic toxicity data is typically given as workplace exposure limits, under the reasoning that if you work with something every day and its concentration is less than the harmfulness limit, it should be okay. Scanning your MSDS, I did not come across any such value, so there should be no serious chronic toxicity if you keep your room well vented and don’t forget to recap the bottle, etc.