A conceded putt is like a gratuity at an eatery. There IS an expectation, but sometimes falsely. There is absolutely no obligation to concede a putt, but here are my own personal guidelines (with the understanding all relevant putts are for a halve or to win the hole):
1. Just about anything outside 3ft is a putt...gotta see it drop.
2. depends on the course and even the hole. It helps to know where you're playing intimately.
3. If you're being a pr1ck, don't expect anything.
That's about it for me.