I check frequently. My diaper change is approx 4 hours max, unless they soil themselves. When we are home, my kids either wear a singlet with diapers or rompers. Can tell easily if the diaper is saggy when full.
The sizes and wt indicator is just a guide to me. For me personally, if the diaper doesn't fit nicely on baby for 1 reason or another, I will change.
Some bbs have chunkier thighs than others, so you'll need to find a diaper that has a bigger thigh cutting. I think mamypoko is 1 of those. Also, do note that different diaper brands has different sizing. Eg. Huggies Ultra has a smaller cutting than Huggies Dry Comfort, Drypers' S is smaller than Pampers' S. etc.