Just noticing your "avatar" suggests that you are a female entering the program. Just to add a little extra info, female Marine Options usually attend the 2nd increment of the NROTC Bulldog/PLC combined class of OCS -- which meets the 2nd half of the summer. It normally starts after the 4th of July and ends 6 weeks later in mid-August.
There has been an effort to possibly open up females being able to attend either the 1st or 2nd increment (not solely the 2nd), but not sure how successful that was. I've heard that last year, the 2nd increment filled up -- which actually opened up some of the NROTC females to attending the 10 week class post graduation, commissioning after completion of OCS, and then immediately reporting to TBS. This is the exception, but nonetheless, it's always in play.
I have also known males Marine Options attending the 10 week course post graduation -- which is usually because of things like injury, probation, etc.
The general rule is, however -- 6 weeks during the summer after your junior year. It will be combined with the "PLC Seniors" who are attending their 2nd time at OCS.