How long do I need to use Mederma stretch mark cream for effect?

No definite timeline. Stretch marks, or striae distensae, behave like scars in that they are the result of skin expansion that stretches the skin leading to skin injury with a thin, atrophic scar. Scars creams, like mederma, may help with some of the discoloration, especially if used for three to four months. Generally, discoloration should improve anyway as the stretch mark matures and lasers may help down the road.