In a large bowl, whip heavy whipping cream with sugar until somewhat stiff peaks form. Be careful not to overwhip it into butter! Set aside.
In another large bowl, whip condensed milk on medium speed until it appears fluffy. Slowly add freshly squeezed lime juice (this was the juice of 4 limes for me).
Continue to whip until the mixture is light and well-combined.
If you microwave each lime for 15 seconds before juicing you will get more juice. I don't know why, but it's true!
Gently fold the whipped cream into the lime mixture, being careful not to squash the fluffiness.
Spread on top of cooled graham cracker crust and refrigerate at least two hours to set. This can even be made a day ahead of time.