"How do you know when it is time to add a sub-panel rather than crowd the existing box?"

Usually when there are no spare slots for new circuits in the old box.

If you have a 200 amp service in a moderate sized house it is unlikely you need more.

Circuits rarely operate at anything like there rated capacity, so additional breakers are more a convenience in not taking down the power over large areas of the house rather than anything else.

The next question is what rooms are you remodeling?
A living room or bedroom usually isn't a big thing, but code changes have occurred to kitchens, bathrooms, etc...

Further, you'll need to find out if the remodeling is such that they want you to bring things up to the current code which may mean things like Tamper Resistant receptacles and AFCI's.

I think I actually have a 400 Amp Service, two 200 Amp boxes. The house has a sauna, well, heat pump, two hot water heaters and a 240 welder outlet. Some of the breakers are about 1/2" and others are 1/4". So there is still some room.

It is the kitchen I'll be remodeling and I am paying close attention to "Code Check" in my planning along with manufacturers requirements. I will be removing two aluminum circuits, oven and cooktop. Most of the circuits I'll be able to relocate, like the two 20 amp circuits.

I was told by the permit people that I didn't have to update other areas, except the smoke alarms.