We report on the fabrication of planar Bragg gratings in polymer substrates and study the impact of the UV dosage and a subsequent thermal annealing on the reflectivity of the gratings and the full width at half maximum bandwidth of the reflected spectra. In addition, the influence of the grating length is investigated, showing that gratings as short as 4 mm continuously exhibit good reflection properties, facilitating miniaturized sensor designs. Moreover, we highlight that the polymer Bragg gratings exhibit a remarkable stable reflected spectrum for over two years. Finally, the experimentally determined spectral characteristics of the Bragg gratings are compared to simulated results revealing excellent agreement.