There is some disagreement on the proper length of a bath, but it falls somewhere between 15-30 minutes. You run the risk of severely drying out your skin if you stay in too long. Wrinkled fingers is a good indication that you should start wrapping things up.  If you do take an extra long bath, make sure you moisturize as soon as you step out of the bathtub.  Bath salts can ease sore muscles, but quickly dry out skin. Keep your bath short if you're using salts. Hot water can dry your skin, but it's actually soapy water that can do the most damage. Soap can strip the natural oils from your skin, so you may want to use a body wash or gel instead. Otherwise, just wait until the end of your bath so you're not sitting in soapy water for at least another 15 minutes.  Look for bubble bath that includes a hydrating oil, or add oil to your bubble bath to keep your skin from drying out.  Use a superfatted soap, which contains a lot of oil and will moisturize your skin. (optional) Again, there is a dispute over whether it's better to shower before or after bathing. Showering beforehand makes it easier to exfoliate and means you're already nice and clean when you soak. A post-bath shower will help you rinse off any oils, masks, and conditioner that may linger on your body. Wet skin acts like a sponge, so using moisturizer immediately after your bath means your skin will absorb as much as possible. Gently pat your skin with a towel and avoid harsh rubbing, which can irritate your skin and may wipe off your moisturizer. Try coconut oil, shea butter, or cocoa butter for ultra-moisturizing action. "Butters" and "oils" are far more intensive than "lotions." Taking a moment to wipe off oil and excess moisture will go a long way in preventing the build-up of soap scum, grime, and mildew. Give the tub a quick rinse with clean water, then use a clean, dry squeegee, microfiber cloth, or soft sponge to wipe it down.

TL;DR in Vietnamese:
Tắm bồn không quá 30 phút. Không dùng xà phòng hoặc chỉ nên để đến cuối. Tắm vòi hoa sen thật nhanh trước hoặc sau khi tắm (tùy chọn). Thoa dưỡng ẩm và thấm khô người. Xả nước bồn tắm và dùng khăn sạch lau khô.