It depends, on a lot of things. Environmental conditions like heat, sun, watering. Also variety of winter melon. You can pretty much leave them in the garden until you see the vines start to die off, then you know they're not going to grow any more and you can store them in a cool, dry place.

I grew one last season that gets to about 20 lbs, so it is a small one! I got 5/plant, and would have had more, but it was late starting, so this year I started one inside, and also have a couple smaller ones (sort of hard to use up 20 lbs, even though it is very versatile). The ones I picked that were 18-20 lbs were about 14" tall, and 10" in diameter. I actually have one now that has lasted the entire winter, spring, and now the summer, with the layer of wax getting thicker as it sat. Just did it to see how long they keep, and I was not expecting this!

Dave

Thanks, violet and Dave.
I have a lot of reasons to be concern about my winter melon. This is my first year growing them and it has not been easy. Of the three plants that I have two of the main is broken, just because I was moving them a bit. I learned it the hard way that winter melons are very delicate. Here is another example. One of the small fruit that was under 1 lb was growing at a spot that would not continue support its weight. So I adjusted it a little and the fruit felt off few hours later.
My only fruit left has been growing very rapidly the first ten days, gaining about 7-10 lb during that time and it has not gained much since, just 2-3 lb in last 7 days (it's about 20 days old and 12 lb; 11.3" long and 5.3" in diameter). I want to know what happened. We did have a heatwave that passed. Was that it?
And I am also unable to estimate its final weight because of its abrupt change in rate of growth.

I never had that problem moving around the melons; in fact, I had some hanging on a fence that I though would fall off that never did, and were over 12 lbs when I finally cut them off, only because it was getting wedged in! They did seem to slow down toward the end, but that is normal with many vegetables, as the energy is being used for creating the seeds. The variety I have this year is supposedly only up to 10 lbs, and they are all on a fence, and I did notice that the stems were much thinner than the winter squash on the fence, so I am keeping an eye on them, in case I have to support them. This is about the time mine started last season, so they are later than the squash, again, but only slightly later.

Dave