Maybe my answers wouldn't fit to your question that much, but they could be helpful anyway!
In some situation you might be asked for an appointment time and instead answering you may ask (this option is quoted by Pollitizer):
Wann passt es Ihnen (denn)?
Wann würde es Ihnen (denn) passen?
Wann haben Sie Zeit? ...
Wie passt es Ihnen denn?
Sometimes you may have the possibility to switch tasks or have more or less free time and want to check if the time that fits the person you'd like or need to meet fits your need too, so you may say:
Machen Sie, bitte (rather polite option)/doch ein Vorschlag?
In the same situation you could also use:
Ich bin (zeitlich) flexibel.
Ich kann mich (zeitlich) auf Sie einstellen.
These are similar sayings to "zeitlich ungebunden" (quoted by user unknown)
Or you could say:
Wann immer Sie möchten.
Wie es Ihnen passt. (also quoted by Pollitizer)
Note that the last two may also, say that you'll have nothing to do or no plans at all, but also show that you are interested in the appointment, so if you are meeting for business it might have some negative touch.