If you just bought all expansions and the 2nd barrack i doubt you will be needing space for the longest time to come and would say use at least 30 on premium and then decide what to do with the final 5 - either do another premium or spend them on charisma/stamina to powerlevel any characters you want if there are any.
I still don't have the third barracks because i'm not in need of it yet. You might as well wait for the next time you have enough SC and buy it at that point instead.