There are a lot of variables here, and I'm mostly just addressing the latter years of dueling, from the late 1700s onwards. I've written previously about the etiquette of a duel, which I'll repeat here as it will help with the answer:

There was a very rigid procedure to dueling, and various dueling codes came about, the most famous being the Irish Code Duello, which was very popular in the late 1700s through the 1800s, and from which I draw most of this.

Now, lets say you insult me, and I challenge you to a duel! As the challenged party, you may choose the weapons. The traditional weapons are swords or pistols, but uncommon ones were sometimes chosen. If you chose swords, and I am not a swordsman, I can request a different weapon, but must accept the second choice.

Now, there are all kinds of minor rules which I'm going to gloss over and instead cut to the chase. The underlying idea of the duel is to satisfy honor. Seconds are chosen, and generally, the seconds are going to attempt to stop the duel by agreeing to terms under which you, the offender, can agree to apologize to me, the offended, to which I would agree. This will go on right up to the point when we are standing en garde. But, once at that point, it would be considered extremely bad form for you to now apologize.

The common trope is a duel to first blood. This was not the case. To agree to a duel to first blood was very poor form. Rather the duel was at least to first blood. If with swords, once the duel began, you can't ask to be pardoned for your offense until I've drawn blood, otherwise you'd be in very poor form. If with pistols, you can't ask for a pardon until shots are exchanged. The duel would continue until I, the offended party, either agree to accept your apology, or else have decided honor is satisfied. In many cases though, the offender merely showing up to the duel would be enough, and no fight would even occur! On the subject of first blood, while agreeing to first blood was poor form, that isn't to say there wasn't an implicit understanding - especially by the mid-1800s, that it was the point the duel would end, but still, to vocalize that was frowned upon.

Now, what that all is to say is that a duel might be very quick and end with nothing more than a pin prick, or it might keep going and going until one (or both) were too bloody and incapacitated to continue the fight, or else dead. The biggest part of what we see in the movies that isn't true is the fancy swordplay. Thats a good way to get stabbed. In a real duel, the biggest priority is not getting killed yourself! Because duels happened well into the 20th century, we actually have some real ones captures on film, which might give you an idea of how they were fought. Here is one from 1967 and here is one from 1958. The second one is of particular interest as it shows something that became pretty common by the late 19th century. After blood was drawn, the action would be stopped, the blade resterilized, and the wound bandaged if needed. While obviously there was still, in theory the chance of death (and some duels certainly had that intention), it kind of illustrates how much it was just about honor for both parties, and not about the ultimate revenge. While when one swordsman was clearly the better it might not last very long, more generally a lot of the duel was spent on these small, positional movements and light feints.

So TL;DR Duels went until honor was satisfied, which could be a single wound, or a dead swordsman.

I've also written a piece about some 20th century duels you might find of interest.