On the ground, CAA regs that one crew member must be in the vicinity of each pair of doors. Ie they must be able to get back to the door if an evacuation was called. The best guideline is 6 seat rows. If your airline allows you to be further than this and you feel uncomfortable you wouldnt be able to carry out an evacuation (do you want to get trapped in the cabin whilost passengers figure out how to escape with their bqgs etc?) then speak to your safety managers.