In general you should start seeing swollen tubers soon if it will produce any at all, only time will tell

These cuttings are 3wks old and you can see the tubers starting to form.
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These will not form tubers even though it's from a caudex forming parent.
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Do you mean 7 week old cuttings? That's how the photo's labeled. Is that typical tuber growth for a successful (tuber forming) cutting of that age, or is this species an extreme example?

I'll see if I can periodically post photos if these guys appear to be showing progress. I'll try to get some in soil in a bright location outside--I'm sure in water on a dim windowsill is not ideal.

They're 7wks now but the photo was taken at 3wks. I think this is an extreme example, Cyphostemma cuttings produce tubers quickly. I have some woody Bursera cuttings that are about 5wks old, will see their roots look like. Also could try to root some Kedrostis or Ibrevillia, same cucurbit family.

Did you root yours in water?

Yep,

I rooted them in water and they're still in water (started 8/11). Is this a problem? At the time I wasn't hoping for anything more than propagating the things. I'm sure they would root in anything and I have plenty more to play with--in fact I have vines dragging on the ground that I need to cut immediately. I do seem to remember that roots in water are structurally different than those in soil (but that they transition back relatively quickly...). I was going to pot up a couple rooted ones today. I'll put a couple new ones in soil while I'm at it. I'll stick them in 50:50 potting soil:perlite. Let me know if there's a more appropriate mix and I'll move them.

WOW! This thing's a monster!

After asking about the mother plant for so long I finally decided to look for a tuber. The plant is in a hanging basket 7 feet (2 m) up in the air and I thought a tuber would be buried a bit, so I didn't want to disturb it.

I found that the tuber was right below the surface. I moved my finger straight out from the stem in just one direction. Finally 2.75 inches (7 cm) out from the stem (just under the top of the soil) I reached the end of the tuber. So if the tuber formed a circle around the stem (I'm sure it doesn't), the radius would be 2.75 inches. I guess I rubbed it a little too hard--it looked like a potato missing a bit of skin (there's no question it's a tuber). Of course I don't know how far the tuber extends in other directions--and I don't plan to find out in the near future--but suffice it to say that it has grown quickly.

This is a seedling which I assume was germinated in the winter or spring. I bought it much smaller a month and a half ago in a 4 inch pot. It's now in an approximately 2 gallon (8 liters) hanging pot and it grows several inches a day from several vines.

Even if I "only" got a 2.75" tuber that's not bad. If its as big (or bigger?) in all directions that would be pretty amazing.

And obviously even if cuttings produce tubers, but they are significantly smaller than these they may still be worth growing. I bought my seedling at Annie's Annuals in Richmond, CA (north of Berkeley). They don't seem to sell in online--I don't know if their store still carries it. Silverhill (S. Africa) lists seeds in their catalog. I know of no other sources. I looked around quite a bit to see who carries it when I was trying to find info on these; they are very hard to find.

I just wanted to follow up on the above posts.

Three cuttings from September--unfortunately I didn't write an exact date down, but they are definitely between 4 and 7 weeks old. These were rooted outside in soil:

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What I assume are small tubers have formed from the roots. These are independent of any swelling of the stems (which does not seem that prominent). The cuttings have not grown significantly during this time. Does anyone have any thoughts as to whether or not these are indeed small tubers? I would assume yes, but I'm interested in input. Could they be only swollen roots, and do I still have to wait a while to be sure? I'm looking for best guesses if it comes down to that. Remember, these are less than two months old, and there's been little vegetative growth.

Here's part of the larger tuber from the parent plant (This is a seedling I bought in a 4 inch pot in July). The tuber is pushing above ground (maybe an inch). It extends about 4 1/4 inches from the central stem at its widest (it's very irregular, and doesn't seem to extend at all in the opposite direction):

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And finally, it bloomed. It's a he--I didn't measure the flowers, but they are about 1 inch.

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Finally the tuber, as I showed, is pushing above the ground. I haven't dug it up yet (and I have no immediate plans to, even if it goes dormant). It's clear that the shape is irregular, and pictures of others in the genus seem to show elongated tubers. Is there any chance the tuber has hit bottom and that's why it's pushing up? Or is it more likely that it naturally starts to grow above ground?