I know cooking meat at higher temperatures will cause meat to lose moisture as the proteins "constrict". This is well known. Cooking a steak sous vide at 160F will lead to a much drier steak than cooking it at 140F. The 160F steak will therefore be much lighter when removed from the bag.
How does sous vide cooking time affect moisture of meat? Imagine cooking one steak at 140F for 1 hour and another for 5 hours. Will the 5 hour steak be drier and weigh less?
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Imagine cooking one steak at 140F for 1 hour and another for 5 hours. Will the 5 hour steak be drier and weigh less?
Briefly: Yes, it will be drier, and yes, it will weigh less. And no, 140F isn't the tipping point, though higher temperatures definitely will dry things out faster. Moisture loss will increase with increased sous vide time even at 131F (the minimum temp I'd recommend for long cooking). It likely will happen at even lower temperatures too.
For one set of experiments in this regard, see these two posts, which compare moisture loss with different duration, different temperatures, and also a comparison with a reverse sear/oven method.
To summarize the relevant bit:
Those results in juice release also mirror observed weight loss in the meat. The 100-minute steak was 90% of its original (before sous vide) weight when pulled from the bag, while the 5-hour steak was 84% of its original weight when pulled from the bag.
I've observed similar patterns in my own sous vide cooking. While there's a certain minimum time needed to bring a thick steak up to temperature and get the juices moving, going past that time will just result in a drier steak and less juiciness. The effect is much more pronounced (in my experience) with ground beef, which seems to dry out quite a bit even at low temperatures with prolonged sous vide cooking even at low temperatures (likely due to the greatly increased surface area that can release juices).
I had hoped that Serious Eats would weigh in on this, but interestingly they don't. In that link, they note that the meat changes texture with long cooking, but claim that a 24-hour steak "is still plenty juicy (a steak cooked 24 hours loses barely any more moisture than a steak cooked for one hour)" but there are no accompanying stats to explain that claim or how much "barely any more moisture" might be. I've personally noticed that steaks seem to get drier with more liquid in the bag with long cooks, so now I don't tend to go past an hour or so myself unless it's a very thick cut or I deliberately want to focus on tenderness over juiciness.
EDIT: For those who want more than a random blogger's word on this, or who question the metrics in that post for how "juicy" a steak might seem to be when you eat it, here's a scientific paper showing that perceived juiciness decreases with increased sous vide time (even at low temperatures). As that paper has a paywall, there's a summary of the findings in this blog post.
EDIT2: In response to a comment that the sample size was not large enough in the blog post, here's an entire master's thesis on cooking sous vide, which shows that "cook loss" (i.e., moisture loss during cooking) increases with sous vide time. One of the studies (n=14 samples) in that thesis shows a statistically significant difference in longer cook times at 55C (131F): after two hours, cook loss was about 14%, whereas at 10 hours and 30 hours, it increased to about 19.5%. The second study (n=40 samples) compares shorter times and various temperatures and does not show a statistically significant (p < 0.5) trend at 55C for 2, 4, 6, and 8 hour cooks. But the graph still indicates an overall trend toward greater moisture loss with greater time past 2 hours even at 55C. For higher temperatures (60C/140F or above), the increased cook time definitely increases moisture loss significantly, from about 14% at 2 hours to about 22% at 8 hours at 60C/140F. At 70C (158F), the effect of time is even greater, increasing moisture loss from about 20% at 2 hours to about 32% after 8 hours. The literature review toward the beginning of the thesis also lists several other published studies that have shown similar trends.
I also would note that cook loss only indicates that less moisture is present in the meat. Perceived "juiciness" is a somewhat different thing, as that depends on free moisture that can be easily released during eating and chewing. Which is why I also linked a study in my first edit above about the perception of juiciness (which declines with increased sous vide time), along with the actual decrease in moisture in the meat.