My rule of thumb is to have same sized container compared to the leaf-mass of the tree. So far it has worked on my citruses and other tropicals.

If that plant is 6 weeks old, and if it keeps growing at the same speed, I'd probably re-pot it in 2 months or so.

It's just how I do things, and I'm quit new to citruses so.. You know :)

I cannot tell you how long they can stay in those cups. I am trying to find an answer to this question by experimenting with my own seedlings. I have found that waiting to transplant until roots show through the drainage holes at the bottom is not a good idea because by then, the roots have circled around the bottom of the container. To prevent the circling, I am trying germinating the seeds in these containers because they are deep: https://www.amazon.com/gp/product/B00USMLNW4/ref=oh_aui_search_detailpage?ie=UTF8&psc=1. I will then transplant into 4"X9" Treepots. (http://www.greenhousemegastore.com/product/treepots/tree-care)

@Jontte Kinnu: Thanks for your advice, I'll keep that in mind!

@ Vladimir: How old are your seedlings? Can you post a picture maybe? I'm always very curious to see other seedlings :)
And thanks for the tip on the Treepots!

I would pot up. It's summer. It'll grow better in a larger pot given good drainage.

Generally the time to repot is just as soon as the tree's root system has developed enought so that it can hold onto the medium when it is sipped from the container, but before root crowding.

OJ, I agree with you but how can one catch it before root crowding occurs? The answer is to use clear plastic containers (drinking cups).

Most growers know from general experience when it is getting close in time to re-pot. Let the medium dry a little, then remove the tree from the container and examine the root structure. Clear containers, allow interior algae growth to develop..

Cover the clear container with aluminum foil to prevent algae growth.

Interesting point about the different mixes. Sounds like a good thing to experiment with.

Vladimir, I even start my seeds in 511. They do fine. I just made sure the 511 is well saturated before planting in it.

Cory

Cory, I don't know if it will make any difference whether I use seed starting mix or 5-1-1. We shall see. (The 5-1-1 was well saturated before use.)