The short answer is that there's is no right time. Every guy is different. But there are ways to know when to switch from short-term mating to a long-term relationship. Getting tired of sleeping with women that you don't care about is one of them. This decision is best made by emotion, not logic and reason. Listen to your gut. And remember you can always go back to short-term relationships if you want. 
Tucker starts this episode talking about how he knew it was time to make the switch in his 30s when he felt lonely having sex with a ton of women. Nils and Dr. Miller give more cues to look for and bad situations to avoid. They also suggest looking at medium-term relationships as training for a long-term relationship. It takes time to transition from short to medium to long.