What were the cues that your kidlet was ready to switch from size 1 (slow flow) bottle nipples to size 2 (medium flow)? Did you just follow the nipple month suggestions (+0 months, +3 months, etc.)?

DD is 4 months ish and we just bought size 2 nipples today. Wondering when to switch. I didn't pay attention to the packaging with DS and didn't move to size 2 until 6 months+.