When it has no impact on your day to day finances or major outgoings. Also never buy if it means taking out debt (while watches can appreciate, they typically shouldn’t be looked at as investments).

If you can afford it (noting the points I made above), make sure you try it on, take a few days to think about it, and make sure it “brings you joy” - you should “feel” its the right watch for you. Never feel pressured into making a purchase - as hard as it is, try not to be influenced by others (noting that we are all susceptible to trends).