15 needed? Afaik you need 200. At least that is the intended amount.
That still sounds decent:
365d / year * 3 fragments/d = 1.095 fragments annually
1.095 fragments / 200 fragments = 5,474 ~ 5 buildings annually. Still fairly interesting production to reward those who managed to maxing out this building and login daily to gain on average 5 each year or on average a building every 2-3 months.
Disclaimer this is btw not taking into account potential more fragments obtained through the year. Just purely daily collection, year round. As long you don’t missing out on 31 productions through the year, it’ll produce 5 buildings annually. Seems to me a fairly decent building with some potential.
Certainly one I’ll be going for. For startups and young cities this building might be the most valuable event building ever released so far. Even with the intended 200 fragments.