What you are referring to is turmeric powder, and there are no specific amounts or ratios of turmeric and lime that need to be used. You simply need to use enough of each ingredient to create a thin paste that can be easily applied to your skin. Normally, for approximately 100 ml of lime juice, you would put around half a teaspoon of turmeric powder. This is not a strict measure though - you can vary the quantities to create a paste that you are comfortable using.
Regarding the length of time for which you should leave the paste of turmeric powder and lime juice on your hands - a minimum of fifteen minutes is advisable, but the ideal length of time is around half an hour. After keeping the paste on your skin for half an hour, you should wash it off, preferably with plain water. It is best to avoid using soap, as it will dry the skin and negate many of the other beneficial properties of the turmeric. Although soap will not affect the skin lightening properties of the mixture, turmeric also makes your skin glow and has antibacterial properties that you can benefit from while lightening your skin.
You should note that turmeric powder will probably leave a temporary yellowish stain on your skin. Turmeric tends to color anything it comes in contact with, which is why it is often used as a coloring agent. However, as mentioned, this yellow stain is only a temporary effect, so there is no need for you to worry. More importantly however, you should note that this will not simply "get rid of the tan" magically. For the most part, a tan will gradually fade if exposure to sunlight is reduced. Turmeric powder and lime juice will only marginally lighten your skin.
answered by M W