When my DD was 4 months, she was doing 45 mins naps. They have extended due to my patience and the ssshh. She is now doing 1.45 to 2 hrs, with me going in and giving her dummy when she wakes up at the 40 mins mark and then she just goes back to sleep.
Even though my DD is 5.5 months (but 4 months 3 weeks as she was a premmie) she can only stay awake for 1.30hr to 1.40hr so she is on a 3.5hr EASY. I act on her cues when she is tired (nose and between the eyebrows goes red and she starts rubbing her eyes) and then I put her to bed.