Romans built large scale concrete structures in months, not years. Given that these would require a great many layers of cement, not just one, the upper bound f

Romans built large scale concrete structures in months, not years. Given that these would require a great many layers of cement, not just one, the upper bound for curing time would have to be several days, a week at the very edge of possibility. Keep in mind that concrete usually isn't allowed to fully cure before construction moves on. Most concrete houses only fully set after 2-3 years and truly large structures, famously Hoover dam, can take centuries.