for JC/poly batch, i can confirm its around 6-9 months, with the enlistment letter sent to you around 4 months after your checkup, then another 2 months wait (if you are in the first intake)
for mono-intake (enlisting with ITE/O/N levels cert), only two friends of mine went that route, so i cannot give a conclusive answer. one enlisted after a year of wait from the checkup date, while the other enlisted after just waiting for 3 months. someone please correct me if i am wrong, but mono intake timings will vary greatly from person to person, and not as "standardized" as per the JC and Poly counterparts.