Depends. Bases within 2-4 clicks will support nearby bases within 2-4 minutes. Several factors are in play here and every battle (and world) are different. Level 65 offense vs level 65 defense between top tier alliances with 4-8 support weapons is going to hurt significantly.
The point of PVP is to cause the attacker as much difficulty as possible (typically in the form of repair time), but still allow the attacker the opportunity to win, always. Except in the case of support weapons. If the attacker cannot kill a base in 2 hits and there are enough support weapons nearby, the attacker will need to first clear all support bases before attacking their primary target.