Fat
Oils are a form of fat that is liquid at room temperature. To give a very, very rough estimate, we can check how long it takes before fat is created in a human body. According to a 2012 study, it takes about 4 hours before carbohydrates are stored as fat in a human body.
Now to the non-scientific part. You can massage the following numbers a lot.
Fat is created as a storage medium. It is one of the later processes that starts in digestion. Your dragon can start much earlier with the creation, shaving off maybe an hour or so. It is still something to be done last, as spewing oil isn't required for immediate survival and basically an incredibly wasteful way of offence. Imagine being able to spew your own fat. It took quite a bit to create it, store it and even more effort to get it down again. Suddenly you just throw away 200g of fat in an attack?
The dragon is likely more optimised for the production as well as having more available for such production due to size. I would say 30 minutes to half an hour after eating that a sufficient oil production can be finished for a few bursts of attack.
Do note that you can optimise the process a lot. If you can create a forceful, directional mist of oil, you can increase the available oxigen to a great degree. This increases the chance of ignition as well as a cleaner, hotter burn. You'll also need a lot less oil per attack to be damaging or at the very least be exceptionally frightening.
Doing this can also improve your chances of igniting the oil. Instead of relying on unsaturated oil to spontaneously combust, you only need a small starter after which the flash point is reached, igniting the rest of the oil vapour. The starter can be as simple as a small electric charge as you find on many gas stoves, or maybe some parts of the dragon rub or bang together for a reliable spark. Do note that the spark in this case needs to be a bit bigger with many kinds of oils. Having a stable oil until ignition also means a lot more safety inside the dragon.
If you want to further increase plausibility of a quick generation of oil, you can have specific diets. Sunflower and olive oils have a lot of energy in them. They are more difficult to light than petroleum for example, but burn longer and bigger. This is great if you can ignite it, as it'll be more dangerous for a longer period of time. As many people don't fully understand digestion and the times, we can nearly say that eating olives can go in minutes to the storage chambers.
Different flames
This can immediately make differences between your dragons. Changing oils for different effects of oil. Some are good starters, while others for long burn times.
Summary
Oils are fats that are liquid at room temperature. With plants and probably animals able to make oils, we can glean some insight about how long it takes. I expect 30 minutes to an hour after a sufficient (very large!) meal can restore the reserves as quickest. Do keep in mind that digestion is in all cases a difficult and time consuming process.
I wouldn't count on a spontaneous combustion oil. In nature we can find not too energy consuming ignition sources for a reliable oil fire. With spewing oil we're pushing out a ton of valuable resources anyway, so using a bit more energy to ignite it should be fine. We can go for more stable mixtures of oil that can be very dangerous regardless. With the most well fed dragons you can have the most energetic oils, burning hot and long, or insanely hot and short.
With different oils you can also explain different dragons easily. Changing a diet can also change the oil of the dragon to some degree.
