It depends on radiated power. For standoff jamming, if your jammer has the right antenna characteristics and is powerful enough relative to the radar, then you can jam from horizon, regardless of altitude. Both spot and barrage jamming are power heavy. Plus, radar signals have to travel twice the distance as jamming signals, so radiated power from jammers can be many hundreds or thousands of times lower than radars.
So rather than how far away, what's more important is how close you can get to the radar before the radar can achieve burn through, meaning it becomes too powerful for the jammer to be effective.
But modern EA systems are more interested in deception rather than jamming. Meaning they want to be invisible to the enemy until the attack actually happens. So range is largely irrelevant in this case, since flying tactics can put aircraft right over the radar, like flying below horizon or stealth. But at a certain range you may have to electronically attack the radar to stay survivable, like when you open the bomb bay doors.