The range of an airsoft sniper rifle is the same as a regular rifle (assuming no upgrades have been put into either gun)
The range of the most heavily upped sniper rifle using a non-clone rifle and quality parts, such as PDI, prometheus etc, would probably be 200-250ft, in terms of effective range. Past that (IIRC), its a crapshoot in terms of mother nature.
This is me regurgitating info that i have picked up. Others such as Amos, CDN_Stalker, or Fox would be better suited to this kind of questioning.