How long will your bar of old-fashioned lard soap last?
That depends on a few things:
1. Where you're using it.
If your bar is at a high traffic sink where it will get more daily uses, it will go a lot quicker. I notice that at each of the sinks in my house, the soap disappears at a different rate.
2. How many people are in your household.
If you have a lot of people in your house, your soap is going to get used a lot more. I base my numbers on a two-person household, because that is what I'm working with. Your numbers might be a little different depending on how many people you live with.
3. If your bar is allowed to fully dry out between uses.
This is the most important factor, and ties into number 1 & 2. If your bar is getting used so frequently that it can't fully dry out between uses, it will significantly shorten the life span. To combat this, I highly suggest using a well draining soap dish. At the very least, make sure your soap isn't sitting in a pool of water between uses. If you can't use a soap dish, you can try standing the soap up after using it so that a majority of it is exposed to the air. Additionally, try keeping multiple bars of soap at each sink & in the shower. Doing this allows you to rotate between them for each use, giving them even more time to dry out.
That being said, here are my rough estimates based on my two-person household & where you are using the soap:
How long will a bar of soap last at the kitchen sink?
2-3 Weeks
How long will a bar of soap last at the bathroom sink?
3-4 Weeks