How long should/can I use tri-luma to treat my face melasma?

If you are not responding to triluma you may need combination treatments including peels, topical treatments, laser treatment. In general you can be on HQ containing products for up to 3 months after which you need to take a break for a few months. Nothing is set in some though. 

Your dermatologist should have explained to you that melasma is a chronic pigmentary problem that is best treated by a combination of at-home prescription topicals, such as Tri-Luma and an in-office series of superficial chemical peels. In many, if not most cases, depending upon only one or the other does not lead to a gratifying outcome, as in your case. Tri-Luma is just one of several topicals that may be tried. Others include hydroquinone in higher concentrations, kojic acid, azelaic acid, arbutin, licorice extract and topical (and oral) tranexamic acid. In-office superficial peels include glycolic acid or salicylic acid in high concentrations, Jessner's Solution, or low concentrations of TCA. Typically a series of six to eight peels are necessary to achieve optimal improvement, and once improvement is achieved, which typically takes about six to eight months, then one of the non-steroidal brightening agents mentioned above may continued indefinitely along with sun protection to prevent recurrence.  Professional microneedling along with TCA 15% has also proven useful for melasma. Make sure that you are consulting with a board certified aesthetic physician with experience and expertise in treating melasma and insist upon seeing his/her personal before and after photos. Best of luck. 

Hyperpigmentation from any type needs a combination approach of peels like cosmelan,  lasers like clear and brilliant and aerolase or laser genesis, radiofrequency like venus viva, and constant skin care and sun protection.  Bimontly facials with silk peel dermalinfusion is essential to hydrate, exfoliate and infuse vitamins and antioxidants into the skin.  If you have acne you can do additional treatments and peels to get more improvement.  I suggest a formal consultation with an expert in lasers and peels. Best, Dr. Emer

HQ usually for 4-6 month on, with 4-6 weeks off. Rotate with other pigment correctors. Be guided by your dermatologist. 
Melasma can not be completely cured, however effective treatment are possible. Melasma patients have skin that is very sensitive to UV light. Even the smallest amount can stimulate your pigment cells to produce colour. The mainstay of treatment is strict UV protection- hats, sunglasses and SPF every 4 hours. 
I prefer to use a combination of creams and laser for melasma. My most frequently prescribed topicals include hydroquinone, along with vitamin A creams and botanical extracts. The exact percentages will depend on your skin sensitivity, clinical presentation and your current skin care routine. 
I usually start my patients on a series of lasers (low dose Q switch or Picosure), along with medical therapy. Another laser I use is the Clear and Brilliant in the Permea setting. In some cases I combine glycolic AHAs, TCA low strength single coat peels, and in other cases I use a tablet called Tranexamic acid to help modulate the blood vessels that are associated with melanocytes (pigment cells). Melasma treatments should be tailored to each patient. 
All the best, Dr Davin Lim. Laser, surgical and aesthetic dermatologist. Brisbane, Australia. Cutis International 

Triluma can be used for a few months then a 4 week break.  We usually suggest doing the Clear and Brilliant in conjunction with as well as chem peels. See your Dermatologist to get the right treatment for you.Best of luck...