Hi Antonio,
It is difficult to give an estimation of how long it might take, as it doesn't only depend of the number of rows but also the number of indexes, columns, size of the data cache, etc.
Regarding the access to the data while update stats run, as on BS environment we use only DOL tables, so the update stats should use isolation level 0 , that is dirty reads, to avoid locking the table while running.
HTH,
Regards,
Victoria.