The mass of a star predominately determines its physical characteristics, life cycle, and ultimate fate. Decades of research have uncovered a common initial stellar mass distribution, however, the processes by which stars assemble their mass remain largely undetermined and may have a significant effect on the resulting star and its planetary system. Several lines of evidence suggest that forming stars, independently of their mass and age, can undergo substantial episodic mass accretion events that are short compared to the formation time; however, the predominance of this episodic mode of accretion is unknown. Do episodic accretion events dominate the buildup of a star's mass (Sum{M_{burst}} ≳ 50% M_{star}) or burst star are they less consequential? With PRIMA, we can unequivocally answer this question. Only in the far-infrared (far-IR) is a protostar's luminosity directly linked with its mass accretion rate. While mid-IR and submm observations have demonstrated widespread and substantial luminosity variations indicative of both large and small accretion events towards protostars, emission at these wavelengths does not directly correlate with the magnitude of the accretion events. Through Monte Carlo simulations, we demonstrate that repeated observations of 2,000 Galactic protostars with PRIMA allow us to detect a sufficient number of large accretion bursts to unambiguously determine the primary mode of stellar mass accretion. Protostars are highly clustered, and assuming a minimum map size of 3deg x 1deg, mapping 2,000 known Galactic protostars within ~2 kpc can be achieved through observing a total of 60 sq deg. over 13 star-forming regions. We propose to regularly monitor these protostars on cadences from two weeks to the entire five- year PRIMA mission lifetime and compare over 25 years against archival Herschel Space Observatory (and other relevant) data. We suggest that the first monitoring observations should be conducted with FIRESS FTM to enable meaningful target of opportunity (ToO) high-resolution spectral line follow-up for outbursting targets. Additionally, we anticipate that stacking PRIMAger data over all of the monitoring observations will provide ample polarization sensitivity across these key star-forming regions to enable exciting magnetic field science....