When do you transplant a starter lemon tree plant to a bigger pot?

I have one about 2 ft tall I started from seed last year. It was in a 6" diam pot and I just moved it up to a 10" pot to give it room to grow this summer. I have a lot of plants and in general I repot them when I find they are drying out too quickly between watering. be careful when you repot as the plant will take longer to dry out between waterings and you may overwater it. since there is no roots in the surrounding soil yet the soil will hold the moisture longer.

be sure not to transplant too early (in terms of years not season) because any plant transplanted too early in to too large of a pot will not do great. if the roots are circling the root ball or coming out of the holes in the pot, that shows a need to upsize the pot. if the roots are circling, you can encourage more root growth by diagonally cutting the roots. trimming back some foliage will help the plant to not experience as much transplant shock as it might and be sure to soak your little lemon, pot and all, before transplanting.

Here's a guide to when and how,
http://pistilsnursery.com/how-to-repot-indoor-plants/

According to this nursery, late spring is a good time since there will be more sunlight which would promote root growth.

If you have any more questions, call your local nursery (not the big box stores with a garden center) and ask them. Usually, you pot up when the plant is root bound or over grown.