Plenty of heat and plenty of sunlight is required for good seed production of rosemary.

According to Madeline Hill, the herb goddess of the South, rosemary is very slow growing from seed, and I quote, "It is extremely slow growing from seed, taking two years to reach two to three inches in height," from Southern Herb Growing by Hill & Barclay).
It sounds like your seedlings are right on schedule. Knowing this, a 2" pot of rosemary seems like a real bargain at $2.oo!
I grow four kinds of rosemary, with Tuscan being the favorite, and always start from cuttings. Much easier, much quicker, but of course I'm limited to the varieties that I can buy as plants.

As others have said, I think you're right on track.

As a very broad rule of thumb, the longer the life-span of a plant, the slower it grows. Rosemary can live for 30 years or more, and it grows very slowly in its first couple of years. It will grow even more slowly outside its preferred comfort zone.

After that, it will take off, and will end up quite a large shrub after 4-5 years or so.

Be patient.

Thanks, all, good advice!

From my searches, it sounds like there will be bipartisan love-fest in Congress before my rosemary seedlings take off. Many have advised to start from cuttings.

Can I use supermarket-packaged fresh rosemary sprigs for cuttings? Is rooting hormone adisable?

I don't know about the supermarket rosemary for cuttings. I guess, but don't know for sure, that the cuttings wouldn't be fresh enough to strike roots well. Do you know anyone who has a rosemary bush?
I usually take 6" cuttings in early spring, strip the bottom 1" of stem clean of leaves, dip the whole thing in water, then the bare stem in rooting hormone (this isn't absolutely necessary, but I've got it so I use it) and then put the cuttings in a 50/50 mix of perlite and vermiculite that is well dampened. I use clear plastic soda bottles as "greenhouses" by cutting the bottom off and removing the cap, allowing some air circulation. Rosemary usually roots within a month, and can be put in soil, either in the garden or in a pot.

Grocery store cuttings might work, but I doubt it would be much cheaper than buying a plant, and with a plant, you know you'll have success. In another couple of months, there will be herb plants readily available in both nurseries and big-box stores, and rosemary is common enough that I'm sure you'll be able find some.

I also wouldn't give up on your seedlings...once it's warm enough that you can get them outside in the sunshine, they'll probably do pretty well.

For practical purposes, just buy a plant for about $3. and avoid all the work. Unless you want to adventure and experiment.