things seem to take for ever if one watches it to closely. watching a clock or watching something grow. it is best not to watch to closely.

Looking good

Hm, today I found about 4 groups of new leaves fallen from the tree. They were very small. Is this bad or is this normal? Can this be linked to underwattering?

I would like to repot the other Calamondin, but it just started growing new branches, is it OK to repot now? Here's what I've read "Potting should take place in the Spring or early summer when the plant begins to show signs that it is growing." I guess that is a sign that the plant is growing right? And it should be OK to repot?

Also: how long it takes a new leaf to turn dark green in color? (after it stops growing in size)

Stressful situations will cause leaves to drop. Over and under watering.

New leaves will also drop when a tree is transplanted.

For watering, I would suggest you get a wooden dowel and sharpen one end. When you think a plant needs water poke the dowel into the pot, let it remain for a minute. If the dowel is damp when you pull it out, wait another day to water.

New leaves can take several weeks to fully green up.

So I shouldn't repot tomorrow?

By the way, here's the tree that I'm talking about.

Does it look like it could last in the same pot untill Spring?

Put the pot on it's side and pull the soil out a bit and see if there are roots wrapping around the pot. If so, I would repot.

In the end the tree will be better off if you repot.

I've repotted citrus just about ever month of the year, if not every week, of the year without any difficulty. It is not when you transplant that makes the difference, it how how well you transplant that makes the difference. . - Millet (931-)

How how are we?
Before:

And now:

If I were you I would remove fruit.