Switch to low-medium speed and drizzle the condensed milk a little at a time. Add more until you get the sweetness you want. Once you're satisfied, switch back to medium-high speed and beat until smooth and stiff.
Place a layer of graham crackers at the bottom of a deep 8x8 baking dish. Evenly spread a heaping cup of cream on top of your graham crackers. Then put a layer of diced mangoes on top of that.
Repeat by covering the mangoes with graham cracker cookies. Be sure to cover the entire layer; cut the cookies to fit. Then cover with another layer of cream and top with more mangoes.
You now have two options: you can either (1) freeze or (2) chill the cake in the fridge overnight. Freezing will result to a more ice cream cake-like end product which you would have to thaw several minutes before serving. Chilling will result to a creamy, ready-to-eat cake out of the fridge (see notes).
Tip for whipping the cream - put the bowl and the whisk you're going to use in the fridge for about 15 minutes before you plan to start whipping. This way your cream stays cold longer and easier to work with.
How much condensed milk you use depends largely on how sweet your mangoes are and how sweet you want your cake to be. Adjust to taste.
The graham crackers I use are squares (see video). I used a total of 21 cookies to fill my 8x8 inch baking dish.
If you choose to freeze the cake: treat it like how you would ice cream cake - thaw before serving. And note that the fruit will be frozen too.