Reg.limit as you asked in your post for SSI if it croses threshold limit of RS.150 lacs. but as and when it's turnover croses the limit of Rs.90 lacs the manufacturer have to give a prescribed declaration to jurisdictional ac/dc intimating the same.
generally there is no fees nedded for the regn.
there is no specific time limit ,some one has to take the regn as soon as it crosses the limit of 150 lacs.Other wise penal action will be taken by c.Ex.Dept.