I am far from an expert on lavender, but mine grow with little care. I think they do better in the ground, not pots. And, it sounds like they are getting too much rain and not enough sun. Lavender likes a dry, not too fertile soil. Mine are in a spot that gets full sun all day, in average soil. I have the Munstead variety, since I am in zone 5 and supposedly munstead is hardier than some of the other types. Can't say how the other varieties grow, but mine seems to thrive on neglect.

The above poster is right; they need sun, sun, and more sun, heat and are drought-loving. They resent humidity, excessive rain, fertile and/or acid soils. It is not tolerant of growing in indoor conditions and I suspect even your porch is too shady for it. They are not delicate plants...they just do their very best in hot, sunny, dry, poor soils where most anything else just dies.

They require full sun plus lean & well-draining soil. Yours are getting and retaining too much water and probably need more sun.

You can at the very least, try something that is similar to a bonsai mix in your pots.

FataMorgana

Get the plants from a place that has a 1-year growing guarantee. I don't know what all their guarantee covers (it surely must protect itself from a rabid owner) but at the least, having a plant die within 1 week should definitely be replaced by the store.

If your plant reaches a certain stage where you can take cuttings, then you can try to have more lavender plants that way, also.

What is your potting mix right now? Custom-made or commercial product? There is valid concern that it may be retaining too much water.

The first one (the one that died in the week) was bought at Walmart and the second was bought at a University's agriculture program's plant sale. The one I have now was bought at a local greenhouse.

I'm not sure what is in the potting mix, but it seems to be quick draining, at least compared to what I remember of the others. The container is also smaller for the plant's size, which I've heard helps.

Lately we've been having very sunny and hot days, not a lot of rain. It's soil is pretty much dry now but the plant seems fine. If the leaves start to go limp I'll water it, but should I let the soil dry out like this every time? How can I tell when it needs a bigger pot?

Make sure that the plants don't get too much sun - on the side of the pot. Sometimes, especially with black plastic pots, strong sun can boil the roots on the side of the pot and once the rot sets in there's no hope. Keep some tough plants like daisies in front of the lavendar on the sunny side of the pot.

For OP & others - Instead of Walmart, visit the locally owned Nurseries or Garden Centers. I find they have superior stock and knowledge than the big box places - not to mention you are supporting local businesses. Starting with better plants does help.

Is it possible to put the plants in the ground rather than pots? Some plants, like many perennials, do much better in the ground. This is especially true if you are trying to overwinter in pots.

FataMorgana