I assumed I would see a noticeable difference. How long does it take? Is it worth getting another session? (photo)

In answer to your question, you would typically not see much improvement after a single treatment. Reducing stretch marks via microneedling, or any other treatment, requires a commitment to a fairly lengthy procedure program with multiple sessions over that time period.You can think of stretch marks as being a tear in the dermal tissue. When skin is stretched during pregnancy, body building or weight gain to the point of that dermal "tear", it has lost its ability to organize collagen bundles. The resulting disorganized collagen support presents as a stretch mark.How microneedling or laser helps is by forcing a repair mechanism from a controlled injury (from the device itself). It is this repair process that causes new collagen production. Each treatment session builds upon the previous to help lessen the obvious differences in skin texture and color.But this new collagen can't return skin to its original state. That makes understanding that you will get improvement instead of complete reversal a key point in your decision to treat.Stretch marks do fade somewhat with time and treatment can help. Stretch marks for the most part don't tan, making them more obvious on tanned skin. Sunscreen will help minimize the difference in color by preventing tanning. 

Hi Abner. Stretch marks need a combination approach for long-term improvement. I typically combine microneedling/PRP with lasers (Erbium, Fraxel, Excel V, Pulsed Dye Laser, Laser Genesis, Yellow laser) or radiofrequency devices like Intensif, Fractora, Skinfinitiy, and Venus Viva. Stretch marks can't officially be "cured," and it will take a combination approach and consistent treatment to see great results. Luckily, there are a multitude of treatments currently available that can be utilized. I recommend seeing an expert for a formal evaluation. Best, Dr. Emer

Stretch marks are very difficult to eradicate. Microneedling with PRP makes them less noticeable. You will need many sessions perhaps 6-7 spaced 3-4 weeks apart. After just one session you cannot see any difference. It takes weeks for collagen to develop. Microneedling is a medical procedure and should be performed by an experienced physician. Also the instrument used and the depth of the needles are paramount. 
Good luck.