sir tell me cat frm.for cast cerificaton how long old

Generally they prefer a caste certificate which is a recent one. If you go through the guidelines their you would find explicitly mentioned to get a latest caste certificate and that too in the format mentioned in the brochure.

If you already have a certificate in the same format as they have asked but it is older than a year then you can get it renew which can be easily done and if not then with the help of your original certificate make sure you get a certificate in the format asked because that will be the only one which will be valid.

If in doing any of the above procedure you are taking time then you can submit your current certificate now and get the correct one before your interview round of any college because all your certificates are manually verified on your interview day. There won't be any problem but make sure you get it done in the time limit I mentioned above.

All the best!!

It depends for which caste category you are asking for.
Refer the Government of India website www.ncbc.nic.in for details.

It shall be a year period. But refer to the official site for updated norms.