my dd is a little older than yours, but we had the same probelm when she was younger. she would nap for 30-45 mins , then could only sustain a 1.5 hour wake time. you will eventually have to actively lengthen that awake time for them. for us, we moved to 1 hour 45 min at about 5 months, then 2 hours by 6 months. she was a little tired at first, but quickly adjusted. at about 6.5 months, we moved to 2.5 hour awake time, no problem. the best part about keeping her up longer is that she then started taking longer naps. usually they are 1.5 hours now. i stressed so much over her short naps, but should have spent that time enjoying her. it all worked out in the end!