ninja 250 consider as big bike only.. Peep tend to get offended when u call the ninja250 as superbike 
Anyway, i u earn around 2.5k, no car, no house to pay, then u can start riding a ninja250 already... Its not that exp to own n to maintain compare with 'real' superbike... If u want to go to the supersports route(zx6,zx10,r6,r1,cbr 600&1000) or any other 4inline 600cc above bike, then u need to earn min 4k, witout car, house n other loan to service also, because maintaining it can be more expensive than maintaining a myvi or any other entry level japanese car...