Compassion is always a choice, and a beautiful choice - as long as it is true compassion, rather than some manipulated response. I think the whole point of compassion, as WillyNilly spoke of, is that it is NOT an obligation.
If you feel threatened/pressured/obliged/like you "are supposed to" give someone your time/attention/some slack, then that is not compassion. Compassion comes from a person who feels safe, whole, and intentional.
The more experienced you are at reading your own cues and warning signs about a situation, the more confident you can be in your choices to enforce a boundary or give someone a little leeway.