They are quite hardy too, down to near -10 C. for short durations. Banana Joe

Congratulations! It sounds like your little kumquat is very happy and doing what it's supposed to. After they sprout and make an initial small flush of growth, then tend to not do much new growing until the following spring, so it sounds like you're doing everything right.

I'd just give it plenty of sunshine indoors, and then maybe some light feeding in late winter/early spring when it should make a new flush of growth.

I've got a variegated-leaf 'Centennial' kumquat in a pot and it really only makes a flush of growth in late spring/early summer, and then ripens that growth and blooms in summer, and then sits there ripening fruit in winter, so I pretty much just count on a single big flush of growth each year.

Like Banana Joe said, they're quite cold-hardy when mature. I have mine outside on the deck (under the eaves so it's not constantly getting rained on) and it seems really happy.

Sounds like you're doing everything right and your little plant is doing just what it should.

Keep us posted and keep an eye out for scale insects as they adore citrus when it's grown indoors.

Grant