According to the Wiki:

After 24000 ticks have passed since the world is created, the game attempts to spawn a wandering trader. If there are no wandering traders currently in any loaded chunks, the game tries to spawn a new wandering trader after every following 24000 ticks. The first spawn attempt has a 2.5% chance of success; if it fails, after another 24000 ticks another spawn attempt is made with a 5% chance of success. If that fails, for all subsequent spawn attempts the chance caps at 7.5%. On average, it takes 14.325 Minecraft days (or 4.8 real-life hours) for a wandering trader to spawn.

It would be nice if a tooltip can be added that shows how much time is left until the next spawn attempt, and the success chance of it. It would be especially useful for skyblock playthrough, but also useful for normal survival.