With present archaeological evidence, it's reasonably assured that significant settlement (including large urban sites) had begun to occur close to 1500 BCE/BC at the very latest. The known and named kingdoms of Iron Age South Arabia, such as Sa'ba, Himyar and Qataban, are all believed to date from c. 1200 BCE/BC onwards. However, it is not fully understood if these cultures and settlements directly link to those of the Late Bronze Age, or are a separate development. There are large gaps in the evidence currently available from South Arabia about its early history, despite the cultures there producing textual records. Likewise it is only relatively recently that archaeological exploration of Arabia commenced, and it is possible that evidence that enables better understanding will emerge. Or it might be the case there is a currently unknown urban site which predates the c.1500 BCE/BC date currently estimated. However, c.1500 BCE/BC is the most accurate guess at the moment based on what is known at this time.