It depends largey where he /she is from, but the main thing is that he /she have a VALID visitor visa PRIOR to entry to the US. Th visa will usually be issued for 6 months,and maybe extended at the discretion of the regional CI (INS) office. If you are already planning to marry then once your fiance is here marry within the first six months and then you are legally allowed to sponsor him/her as a family member. You should be aware that even though you are married you will still have to PROVE that the marriage is not one of convenience i.e. to obtain a Green Card.They do have very strict rules on this. I hope this helps, good luck.