The effective range of an AEG varies by model rifle and what bb weights you are using. Stock AEGs can have an effective range of 35-100 meters.
First lets talk about what effective range is. Effective range is the distance you can reliably hit a target about the size of a torso. With a well tuned hop-up a basic AEG shooting around 350 FPS with .20g bbs will shoot straight for about 35 meters, and then lose velocity and start to arc down into the ground for another 10 to 15 meters. The effective range for this rifle would be about 40-50 meters.
To increase the effective range or the range that the bbs stays straight you should consider using a heavier weight bb. The heavier the bb the slower the bb flies but it is also less affected by factors like wind and gravity. Not all AEGs are right for very heavy bbs. Read this article for more info What BB weight should I use in my gun?
You can also increase the velocity by switching to a heavier spring. increased velocity will lead to increased range but not necessarily increased accuracy. Be careful as this does require opening up the gearbox and can lead to you voiding your warranty. Some gearboxes like the Valken Alloy and ASL+ line come with a quick change feature to allow you to change the spring without taking the gearbox apart.