How long does platelet rich plasma treatment last?

Short answer: the results can be maintained. Long answer: Platelet-rich plasma has been utilized for years in wound healing and orthopedic conditions, but more recently it has gained traction in aesthetic medicine, especially with respect to hair loss. Several clinicians have had some success in treating alopecia of various etiologies (lichen planopilaris, alopecia areata, etc), but most of the limited data have been focused on PRP use for male- and female-pattern hair loss.
The mechanism of action involves platelets’ ability to aggregate together, activate, and release alpha-granules containing various growth factors, such as platelet derived growth factor, transforming growth factor beta, vascular endothelial growth factor, epidermal growth factor, fibroblast growth factor, and insulin like growth factor-1 (IGF-1). Their release increases angiogenesis and vascularization, providing resting telogen hairs the signal that they need to enter the anagen phase. This increased vascularization and mix of growth factors also fosters the proliferation of dermal papilla cells and inhibits apoptosis, helping to prolong the anagen phase.More specific to androgenetic alopecia, PRP may help to counteract certain effects of dihydrotestosterone (DHT). In a mouse model of androgenetic alopecia, DHT was shown to block IGF-1 contributing to hair loss and PRP is a known source of IGF-1.
Platelet-rich plasma is fully autologous and thus provides patients a more “natural” treatment, with side effects limited to procedural discomfort and bruising.
There is no standard system, quantity, or interval of treatment regarding PRP for hair loss, which is why we currently utilize a proprietary hyperconcentrated platelet rich plasma concentration and treat patients monthly 4 times, then reevaluate after 8 to 12 months. If a patient has a response, then retreatment at follow-up may result in continued efficacy if performed at a 6- to 12-month basis.

Platelet rich plasma (PRP) is not a cure but it is a great option to slow down hair loss.  Generally, we start with 3-4 sessions over 3-4 months.  By this time, you will start to see some improvement in your hair density and thickness.  Afterwards, keep up the PRP injections every 4-6 months depending on how quickly you're losing your hair.  How long PRP lasts really depends on where you are in your hair loss cycle.  No treatment cures hair loss.  PRP will slow it down but eventually you will still lose your hair, just hopefully years later than expected.  If you are younger, each PRP session will work longer for you, in general.  

While there is no exact answer as to how long PRP will continue to work, in my opinion it lasts for about 6 months. For this reason I have most of may patients on a PRP maintains schedule of 2-3 times per year. Best wishes!