After how long without significant results can you say that a ptosis surgery failed?

After one month you can understand the ptosis surgery results. Bu it depends on the tecnique and how much edema you had after the surgery. sometimes we wait 1 week, sometimes we wait 3 months after the ptosis operation.

Ptosis means drooping of the upper eyelid which can be present in one or both eyes. The good news is ptosis can be corrected with a quick 30-60 minute operation under local anesthesia. Ptosis surgery has a success rate of 95%. Only 1 in 20 people need correction surgery.
It takes approximately 6 weeks for the swelling to go down completely. During the healing process, your eyelids may seem low to you. This is due to swelling and once it resolves the results will be clear. If after 6 weeks your eyelids continue to stay low or high please contact your surgeon for a further evaluation. Best of luck to you.

I presume you are discussing ptosis of the eyelid, which results would be noticeable immediately, even with swelling. At six months, everything if fairly well healed and you will see no further improvement.