That depends on a lot of factors, but the most important one is YOU. I hope you feel good enough about yourself to believe that you're worth loving. Here are a few things to consider in determining how long to wait on someone when there are 7.5 billion other people in the world and only about 100 years to live, if you're lucky.
1) Mutual commitment: Evaluate your relationship understanding and level of commitment. Is this a marriage or long-term, exclusive relationship? On the other extreme of the continuum, have you never even shared your feelings with them (and thus you don't know whether the feelings are one-sided or mutual)? It's very hard to maintain a relationship over a long period of time without a deep spoken commitment. The more mutual it is, the longer I'd personally be willing be invest. If there's any component of secrecy to the relationship such as in a clandestine extramarital affair, beware. You'll probably be waiting forever for them to leave their spouse.
2) Expectations: What do you expect from each other from a behavioral standpoint? What constitutes a betrayal? For example, you might consider "waiting" to include loving someone from afar while continuing to casually date or fall in love with other partners over the years. Your beloved might consider "waiting" to be exclusive emotional and sexual fidelity, even if you haven't communicated for a long period. Know what you each expect. If there's an imbalance in expectations (e.g., one partner can be sexually active with others while another is expected to be exclusive), then that's a red flag.
3) Reason for separation: Is this a voluntary separation in which they had the option to stay with you or take you with them, but instead chose not to? Or was the situation forced, compulsory, not their choice? A variety of examples include: military deployment, job relocation, going to jail, being married currently to someone else, attending different colleges.
4) The details: Is there any timeline or plan for getting the two of you together? How concrete is it? Has either of you articulated "deal breakers" for the relationship? Has either of you violated these? The more concrete a plan and timeline, the more confident I'd feel about waiting. Failure to meet intermediate milestones would signal red flags.
5) Opportunity costs: Opportunity costs refer to what you give up when you choose another option. By waiting for this person, what are you potentially giving up? Are you okay with that? Seriously give this one some thought.
As much as you have a connection with this one person, sometimes the timing doesn't work. Don't pine away for someone during the best years of your life. If it was meant to be, they'd prioritize you as much as you have prioritized them.