It is most common to do CSLIP your 4th summer, but it is possible to do it the 3rd summer, if chosen. At this point, my ds would prefer to go with his classmates/friends next summer instead of this summer with cadets a year ahead, but I don’t think he’ll say no if it is offered to him.
I seriously doubt that a cadet could do more than one semester exchange. They are only offered to first and second class cadets (juniors and seniors) and are competitive. Plus, there are a lot of considerations with completing core curriculum/majors classes and USAFA’s in-residency requirements. DS told me that it is possible to do a summer and semester exchange. He won’t be able to be fluent enough in his language for semester exchange before firstie year so that is his goal. There are also other shorter trips offered throughout the year (like at spring break or summer), but those seem more spontaneous. The ones ds has knowledge of still go through the Dept. of F. Lang.
I did CSLIP back in my day the summer before my first year and it was AWESOME!! I highly recommend it.