For instance.My fiance gets his visa....... whenever.How long do we have to wait to go back to Panama to visit ?

FAQ: 6.7)...Can my spouse leave the United States, and then re-enter, before receiving the Green Card (conditional permanent residency)?

http://www.visajourney.com/faq/k1k2visa-aos.html#6.7

Has to have AP document in hand before leaveing the country.

File for AP at the same time you adjust status after marriage.