Short answer: Days. Consider every English translation since the Vulgate (Latin) Translates the Hebrew "yowm" as "day". Also the text numbers the days and expresses the cycle of morning and evening. Any plain reading should leave the reader with the feeling that the days in Genesis 1 are ordinary days of approximately 24 hours each.

There are many people within and without Christendom that invoke many arguments to allow the forcing of vast ages of time into the creation week. We address it more thouroughly here.