One of the age old questions that people ask in relationships is "when is enough, enough?" When is it time to walk away versus continuing to fight for your relationship? You would think the answer is easy, but due to all those factors that make us human (emotion, attachment, optimism, etc.) it's much more complicated. However, there are some things to look for that can be obvious indicators if you don't ignore them.
Have you ever been in a relationship where it feels like you were putting in all the effort, and the other person was either doing nothing or the bare minimum? One-sided relationships are not fun, can be draining, and can damage the one putting in the effort. So, why do we stay in those situations?
For one, some people feel it's better to have someone than to be alone. And those people are willing to pay heavy prices just to avoid the feeling of loneliness. They don't mind being the only one to put in effort, because in their minds that persons presence is enough. Them being there and providing companionship is all that matters. But, that's a basic requirement to be with someone period, and "shouldn't" count as doing anything special. Besides, you both are providing presence and companionship to each other.
If you find yourself always being the one to initiate phone calls, plan the dates, buy thoughtful gifts, concede more arguments, sacrifice more of your goals and dreams, show more affection, etc... pay attention to exactly how disproportionate those things occur. That could be one of the signs that you are in a one-sided relationship. Reciprocation matters. Even with understanding that everyone expresses themselves differently, there are plenty of ways to show effort.
One way to show effort is simply through appreciation. If you are doing all of those things, the least they can do is show a genuine appreciation. Take the time to say thank you, be present and in the moment with you as much as possible. Listen to you, and have compassion and empathy for the things you express. They should make you feel special, wanted, and loved. I can't speak for everyone, but those things suffice in reciprocating effort to me, if they aren't willing to do those other things.
Speaking of which, another way of showing effort is through effective communication. Especially when it comes to addressing concerns that you may have. When a person really cares about you, they'll take the things you say to heart. When they don't, they tend to get more defensive, or even angry that you spoke out. That's selfishness, not love, nor appreciation for what you do. Communication and attention are two things that require little energy when someone is truly in love with their person, but the impact can be major! If they can't do these basic things then they are taking you for granted, and you should reevaluate where things really stand.
Something we should all know by now is that people will definitely make time for the things/people they want to. And if you find yourself always having to beg for time, and still not getting it, then it's ultimately because they don't want to give it to you. You are not a priority for them, or important enough to warrant that from them. The best you can do is act in kind. Don't give more, or expect more than you are getting. That's one way to keep yourself from getting entangled in something that will just drain you in the end.
You'll know when it's time to walk away when you find yourself continually asking yourself how that person feels about you. When someone is in to you, you won't have to guess, they'll happily provide you with assurance regularly. You'll also know when you find yourself questioning if things are worth it. That means you don't feel that the effort you are putting in is being reciprocated, or at least not enough. When you are getting a "sufficient" return on your investment, you'll never feel this unbalance. Lastly, though there are other signs, you'll know its time to leave when you start to feel emotionally, financially, physically, and/or mentally drained.
You don't have to run right away, but definitely don't ignore these things. Have those conversations, and see if things change or remain the same. Sometimes a person may not know that you feel the way you feel. Or, things may have been the same way for so long, they don't even realize it as bad, but as normal. By having the conversation and making it known what the issues are, now you have a point of reference to monitor their behavior. If they care they'll take your words, and make the necessary changes. If they don't, they'll remain the same then you'll know. Change doesn't happen over night, but the attempts will be apparent.