Roasted peppers will keep for up to three weeks or more if immersed in oil and refrigerated; generally, I add some vinegar and salt, which seems to protect them from spoiling. It's unsafe, though, to assume this as a general rule. Peppers are a low-acid food; the apparent absence of air due to the oil layer on top offers no assurance that bacteria could not enter the food just the same. Storing peppers under oil at room temperature or warmer dramatically increases the possibility of spoilage. The only safe method of long term preservation is pressure canning, which, of course, alters the quality of the roasted peppers. The best and most reliable advice I can give is to make small batches, continue to store them as you have been, in the refrigerator, and enjoy them as soon after you've prepared them as possible.