Subsultures of MCF-7 - (Aug/06/2007 )

Hello to all,
I start to work with MCF-7 cell line from DSMZ, I use DMEM supplemented with FBS 10%, 1% insulin and 1% antibiotic, and seems that all work fine.
But now i need to know how many passages i should do until frozen them, i'm affraid that cell line could lose any characteristics if i let them in culture with so many subcultures! Could any one help me please ????

Freeze them at the earliest possible passages.

normally when i start a new cell line to work on, i'll grow the cells in big scale before i start my experiment. eg. i grow in few 75T flask and freeze them down in many vials (lets say i freeze them down into 10 vials of passage x) --- therefore i get the earliest passage i could.
If i need to thaw a new vial to work on, i'll thaw from the the passage x vial, before i start anything, i repeat the same thing as above and i get another 10 vials of passage x+1.....