It's different for each little one. I would suggest following your los cues. When they start stretching it out themselves you will know they are ready. But you can try 15 minutes at a time to see if they are still ok with their routine or ready to go longer.
I think my lo was 4 months when he went to a 4 hour easy. But it was modified. Some feeds were 4 hours, some were 3.
HTH