10cm (100mm) wide between the front dropouts is the most common size that exists.

99.9% of the new front axle sets will be designed for that measurement (unless you bought one for a folding bike, or certain vintage bikes). Of course, new Thru-Axle standards are coming out quickly, and are different.

As [MENTION=291784]andrewclaus[/MENTION] mentioned, the quick release axles are about 6mm longer than the inside width between the forks, or about 106mm long. When you get the cones and locknuts installed, about 3mm of axle stub extends beyond the locknuts.

The wheel then slips down into the dropouts, and the quick release lever clamps it all into place.

There are a number of caveats that you may run into. One of the biggest is that many threaded axles are English, while quick release axles typically are Metric. So, swapping frequently requires finding matching axles, cones, lock nuts, and perhaps washers, all the right size to match whatever seal you have.

Not a major issue if you have a few spare parts laying around, but it can be a bit of a hassle if you don't have the spares.

Bike co-ops often have cheap spare parts. Or, as mentioned, a good bike shop with a pleasant staff might dig around and find used parts for a reasonable price. Even buying the RIGHT new parts may be cheaper than hit and miss on the internet.

You can still buy the parts and do your own work.

Do you have "CONE WRENCHES"? They are flat wrenches of various sizes, 13mm, 14mm, 15mm... etc all about 1/16? thick. Some people like pairs of cone wrenches. I can generally do fine with a single cone wrench + a 17mm (or adjustable) wrench.