EU data protection laws grant consumers the right to access the personal data that companies hold about them. In a first-of-its-kind longitudinal study, we examine how service providers have complied with subject access requests over four years. In three iterations between 2015 and 2019, we sent subject access requests to vendors of 225 mobile apps popular in Germany. Throughout the iterations, 19 to 26% of the vendors were unreachable or did not reply at all. Our subject access requests were fulfilled in 15 to 53% of the cases, with an unexpected decline between the GDPR enforcement date and the end of our study. The remaining responses exhibit a long list of shortcomings, including severe violations of information security and data protection principles. Some responses even contained deceptive and misleading statements (7 to 13%). Further, 9% of the apps were discontinued and 27% of the user accounts vanished during our study, mostly without proper notification about the consequences for our personal data. While we observe improvements for selected aspects over time, the results indicate that subject access request handling will be unsatisfactory as long as vendors accept such requests via email and process them manually.