It's hard when you have big kids! My DD is 28mths (36") and 33-35# depending on the day you weigh her. My boys were that weight much earlier so there just isn't anything you can do except FF them once they hit that mark.
hehe and hard when you have small kids as well. My DD won't hit 35 lbs until age 7 or so. Obviously US seats are not made to RF kids that long. She probably won't hit 30 lbs until closer to age 5(which is most people here's minimum weight requirement). Again, US seats are really not made to RF super tall leggy older kids. So even getting her to 30 lbs would be very difficult.
So for us, after age 3, we go with our personal feelings. DD1 is 3.5. I will be turning her this spring. She will be almost 4. She will weigh about 25 lbs. She hasn't gained in almost 2 years and isn't going to. She has started to fuss and complain and kids make comments to her and once she's in 4K I really don't want her to stand out more than she already is. Like "hey look at the tiny midget kid who has Aspergers and acts weird and can't talk right AND sits backwards" She has enough on her plate.
Not to mention, very few people have had to wrangle a very tall 4 year old in and out of a RFing seat daily. I know some here have and I give them credit. But for those who had larger kids and DIDN'T RF a 4-5 year old, they really don't know what it's like day in and day out. They just stop fitting well. It's a struggle to contort my daughter with her gigantic legs into the seat and I can't imagine doing it much longer. She says she isn't comfortable anymore either. She is super tall though and it's all in her legs. Tall torso'd kids or shorter 4 year olds may fit easier. But I've found it a struggle and I'm ready to be done.
Now with "normal" kids, it's time to turn them when they outgrow the seat RFing by height or weight. If they have a "40" lb limit seat and outgrow RFing early by height on those before about 2.5 I would buy another seat to RF longer. After about 2.5 I probably wouldn't buy a seat to RF longer
With heavier kids, you have to turn them at the weight limit. My DD2 will be FFing shortly after her 2nd bday which makes me sad
especially since DD1 will RF so long.
With lighter kids, it's a parental decision to me after age 3. I listen to their input also around that time and think of social issues as well.
Hope you find something you can handle!