Quote:
Originally Posted by Ensigm
I see, thank you. For a given number, at what level shall we stop using ECM and switch to NFS?

There are two flavors of NFS: Special, abbreviated SNFS, and general GNFS.
For numbers where SNFS is faster, a rule of thumb is to ECM up to 0.22 * the difficulty of the number; e.g. SNFS300 would get ECM up to 66 digits (about 1.4 * T65).
For numbers where GNFS is faster or SNFS is not possible, a rule of thumb is ECM to 0.31 * the number of digits; a GNFS200 would get 62 digits (2 * T60, or a T60 plus a T60 worth of T65sized curves).
These rules of thumb are not super accurate; to be precise, one gives up on ECM when the marginal next set of ECM curves has less chance to factor the number than the fraction of expected NFSsolving time the set of curves takes. That calculation is tricky, to say the least, so the rules of thumb above exist; both my stated ratios are on the low side, many people do a bit more ECM than indicated here.