The art of knowing when enough is enough is just that - an art. In daily life I often pause, consider if this is the moment to change course and move on, or to fight through and persevere.
Friendship. A roommate from college, once best friends, now estranged. An email, then 2, then 3, with no response. Ask to meet for a drink, ask for a reply. Give up? Maybe after the second, but sometimes I can't take my own advice.