It's going to basically happen overnight. My son stalled out for quite some time, and then bam, grew like a weed and gained the weight to match.I don’t know when to be concerned or not….. my kid just turned 3 in October. While I have seen HUGE strides in mental and emotional capacity and huge strides in dexterity and hand/eye coordination I just feel like she isn’t physically growing. She has been exactly 30 pounds for over a year and doesn’t seem to be growing in height. My wife swears it’s normal but I always hear stories about how kids grow like weeds but I’m just not seeing it. I really want to get to the stage where we can switch from a full car seat to just one of those smaller booster/convertibles. But that’s not until at least 40 lbs. maybe I’m just nuts but I don’t know- she just doesn’t seem to be growing physically as fast as everybody makes a big deal with toddlers