How much is appropriate to tip in Thailand? In what situations is it usually given ?

Do as you please. If you think the other party has done well, you can give them money, for example, you can put 20 Thai baht by the pillow when you stay in a hotel. Give me slippers and key chains when you leave. It is said that giving a large amount of tip in a hotel may have fruit plate. But I haven't tried it.

You can give me a tip after I serve you, 20 or 50.

In most cases, you don't need to give a tip because service fees are usually included in your meals and hotel stays, with some as high as 20%. If you take a taxi and the driver has good service and is not overcharging you, you can give a tip of 20 baht or more. If the service exceeds your expectations and you are happy with it, you can give a tip.

Tips are just up to personal preference. I will give around 50-100.