longest time a circuit can sit in developer
Nobody can answer your question but you.  Only YOU have your combination of board, resist, developer concentration, temperature, etc. etc.  You must establish some consistent "process control" where you know exactly how much developer concentrate you are mixing with some measured amount of water. 

Typically, you can make a strip of board marked off in several divisions. Then you can dip the whole board for 1 minute, pull out the first section and leave the rest in the solution for 1 more minute. Then pull out to the next section for 1 more minute. Etc, etc, etc.

Then your board will have the first section processed for 1 minute, the second section for 2 minutes, the third section for 3 minutes, etc.  Then you can look at the finished "test strip" board and see exactly how your (MEASURED) solution performs with your resist at various times.  Then you can select the time right in the middle of "too little" and "too much".