The rate of accumulation is not something that can be answered here; and although opening windows and doors will cause the accumulation to dissipate that doesn't mean the home is safe (as you know).
Your local fire department should be able to answers questions about carbon monoxide in your home. The potential danger with carbon monoxide is, of course, carbon monoxide poisoning.
With a gas oven, depending on the incident, another concern would be build up of natural gas. Your gas company can answer questions you have about any incidents, concerns, or potential build-up of natural gas. The potential danger with natural gas is explosion. If you believe natural gas is the problem don't do anything that would/could cause a spark.
With either problem, it's possible the incident was minor enough that experts can assure you there's not much reason for concern, but it's important to ask, just in case there's reason to have the home checked.