A novel core dopant called diphenyl disulphide is used in PMMA-based fibers, permitting both increase of the refractive index of the fiber, together with good photosensitivity. FBGs can be inscribed in these single mode fibers within 7 ms using 325 nm laser and exhibit significant growth post-UV irradiation. In the present work, we investigate the grating growth behavior upon long UV irradiation time. Furthermore, we demonstrate the fabrication of fiber Bragg gratings with irradiation time ranging from 7 ms up to 10 s using a 325 nm laser and we investigate their growth post UV irradiation over 7 months. It is demonstrated that all the FBGs exhibit over 10 dB growth within the several month post-UV irradiation period. Raman spectroscopy measurements were carried out for several months on thin films post-UV irradiation, and significant changes in the molecular bounds of both diphenyl disulphide and PMMA are recorded, establishing the UV induced photo-chemical reaction responsible for the FBG growth. Finally, the reliability of the novel core dopant and the potential use of the fiber for in-vivo sensing was investigated by using humidity cycling at temperatures near the human body core temperature.