Don’t be a convenience to anyone! You and your time are valuable all the time not just when someone needs you to fulfil their needs. You teach people how to treat you. If they can call you anytime and you are always available or you are willing to cancel plans just to do what they want they are really not your friend. A true friendship requires both parties needs to be met. That means listening to each other and supporting each other. A friendship requires give and take on both sides.