Every roof repair is different. A roof repair could last months or even years depending on its location on your roof, the stability and quality of the materials used as well as the materials surrounding the repaired area, the amount of storms that occur after the repair is performed, whether or not the roof suffers additional damage near your previous repaired area, etc. There are many possibilities as to what could cause a repair to last a while or need repairing again in the future. The important part is that a problem area is addressed immediately if it is showing signs of leaking or water damage, even if it has been repaired before.