self soothing

M
my baby girl is 4 months old (she'll be 5 months on the 18th). I want her to learn to self sooth but how long should I let her cry before I do pick her up. it's normally around her nap time when she's incredibly fussy. I lay her down when she starts yawning and give her the Pacifier and hum to her but as soon as her paci falls out she wakes up. any advise?