Hello ! Based on your photo , they are coming along nicely , your bud are still in a early stage so I would not be affraid about fattening , they are nice and will get more volume soon. Still based on the photo it's hard to tell you when they will be ready but you can wait for 4 more weeks at least, northern light are good buds producer so I'll wait for big buds to shows up. According to your yellow lower leaves your plants have a slight nitrogene deficiency but at that stage it's not really an issue because at that stage plants mostly needs PK and will use the stocked N by making lower leaves yellowing and fall. If you go 4 more weeks then the next(s) feeding will provide enough nutrient to let her make fat buds. The better way to know if your plants are ready is to get a magnifier and look at the trichomes ratio to make an harvest based on the kind of effect you want ( high = clear and milky trichs , stone = milky and amber trichs). I hope it will help you ππ