How do you know when to water

What soil? We've had multiple posters swear up and down they use "fast draining" soil but when they take the plant out if the pot they find a soaked and rotten mess.

Is the soil completely dry down to the bottom when you water?

Are you re-using the soil that the previous plant died in?

Where are these plants coming from? If they are being grown outdoors and you move them indoors they will drop leaves unless you provide an adjustment period.

Most important.... are you perhaps watering with water from a water softener? If you are, you are putting salt on the plant every time you water.