when to change baby's diaper size

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I'm a FTM and my daughter is 3 and 1/2 months and wears a size 1 I was wondering when is it time to change her diaper size? I hear some people say they wait until their kid has a blowout she hasn't done that they don't seem to be tight on her legs or anything so I'm just curious as to when is a good time to switch to a size 2 she's almost out of her last pack of size 1 I don't want to go by 276 more size 1 diapers if she's going to end up in a size 2 soon!