How safe are flammables as indoor cleaners?
Using any volatile and flammable fluid indoors can pose serious health and property damage if used in a reckless manner. By reckless I should say "not adhering to the warnings by the manufacturer". If the item to be cleaned can not be relocated outside than precautions should be taken to limit the build up of explosive vapors. Allowing outside air to circulate through the room to remove the fumes will reduce the chance of a serious "flash over". It would be wise to note any appliances (water heater, stove, heater, etc.) that have an automatic or standing ignition that could ignite fumes. Also realize that even an electric spark from something as innocuous as an electrostatic spark from a carpet can be enough to initialize an explosion. As for knowing when the fumes have dissipated enough to be safe: leave the windows open long enough (after you've capped the fluid and sealed any rags containing residual traces in a metal container) to allow stale air to be replaced. Use your olfactory (nose) senses; if you smell fluid than it's still in the room.