The only way I can think of is to check if the watch has stopped every hour after maby 30 hours of running, but I don't know if that is the proper way, so I would love to hear what the experienced horologists have to say.
Thanks
Francesco

Wind it up and see. It all depends on the mainspring that is if it is worn the watch will run for maybe 20 hours but a new one maybe 30......what does it matter?, wind it up more often.

When it stops running it will have time. 2:37 that does not mean it will keep time till it stops.

Hi Francesco, First of all I am no expert Horologist, but if I understand your question right this is how I have tested to see how long a paticular watch will run. Wind the watch fully and set the watch at 12 noon and let it run, check the watch in twelve hour increments and log each 12 hours on a sheet of paper, once the watch has stopped say at 7, then go back and add your twelve hour logs plus the 7 and you will have how long the watch has run. This may not be the professional way to do this, but it has worked for me. Most watches have a 42 hour main spring, but that does not mean it will run that long because of the wear and tear on the watch. Just remember to try to wind the watch everyday at the same time and you should be okay. Hope this helps--Tim

To test watch performance (i.e. How GOOD it is) would insist the watch be cleaned and known to function well with a lively mainspring, otherwise why do it?

Besides the dirt and build up of oils the mainsprings may "set". Here is an example of the old mainspring and a new one. The old one will run the watch a bit. The new one should run it correctly. I find most watches that have blue steel mainsprings are set when I remove the old one. Just because it may have been installed 20, or 50 years ago.

Now a good discussion of mainspring health and materials would fit this post well. One of the nicer things I have noted over time is how well Factory supplied Blue Steel Mainsprings work when they are "fresh" (Mike's Picture above is an excellent example).

I have disassembled and serviced watches that were in still running condition that had an UNWOUND mainspring in the Barrel that looks "almost new" even though it is clear they have been in the watch many decades. Such still "healthy" mainpsrings if rust-free are (in my opinion) more desirable for these older watches than a "new" white steel Swiss Mainspring.

On the other hand, when mainsprings are left fully wound in the same watch usually (due to a broken balance or other issue*) for the same period of time, they come out looking like the "tired" one in Mike's photo, those may (just) run the watch again, but should be replaced if one expects any longevity and time accuracy of that watch.

*[Apparently the the mainspring will "relax" under the full wound stress causing this. In the mechanical world there are several names for this but in short it is simply the "plastic flow" of a material left under stress which is temperature cycled over many years time.]

The last great supply of "New" White Steel Mainsprings has been rescued and is stocked at Cas-ker. This is my source where needed. I recommend them for price and quality.

http://www.jewelerssupplies.co...ing-size-charts.html

This is their chart;

I just wind them and remember what time it was when they started. All of mine will run at least 32 hours and that includes the dollar watches. A couple will run up to 38 or 39.

same here as Donald's method
wind them writ down the time and date and let it run till stops and you will know how long.

The ones I've owned and seen run an extraordinary about of time are the 992Bs and 950B's, usually 40 hours plus.

Regards! Mark

I just do what Donald does. I generally am not that concerned how long they run if it is at least 30 or so hours. I figure that winding them every 24 hours at roughly the same time is good enough on any watch I carry.

Mark, I'm not positive but I believe all (or nearly all) 992bs and 950bs have "new" white alloy mainsprings. They seem to last forever. I've never changed one in either watch.
Tese will continue to give reliable service. The blue ones being left wound as a result of not running for another reason, are subject to setting. When they set, they will not give good service. I gave a watch to one of my brothers. It stopped working and I couldn't see anything wrong with it. When it stopped a little turn of the winding crown would send it into action. Later it would stop again. When I finally pulled the spring it looked like the coiled one above only worse. Put a new spring in and that Elgin ran like no tomorrow.