Wouldnt say you can put a definate age on it,all dogs are different and i think you just know when it just isnt working out ,ive just got rid of a well bred lakeland that would go in a stop end but it just didnt want to find its own work,id had him in big places,small places a few times and he came out after looking then put my young laky bitch in and dug her out.A few lads said the penny might just drop but then how much longer do you give it,i think if your honest with yourself you know when to say get rid.