There's no hard and fast rule for when it's too late to wish someone a belated happy birthday, as it can depend on the relationship you have with the person and their personal preferences. However, as a general guideline, it's best to do so as soon as you realize you missed their birthday.
In most cases, a belated birthday wish is better late than never, and it's a thoughtful way to show that you remembered their special day even if it was after the fact. Here are some considerations to keep in mind:
Within a week: If you're only a few days late, it's still acceptable to wish someone a belated happy birthday. Within the first week, your greeting is likely to be well-received.
After a week: If more than a week has passed since their birthday, it's better to acknowledge that you're late in your greeting. You can say something like, "I know I'm a bit late, but I hope you had a fantastic birthday!"
Close relationship: If you have a close relationship with the person, they may be more understanding about the late wish, and it might not matter as much.
Casual acquaintance or professional setting: In these situations, it's still polite to send a belated birthday wish, but the longer you wait, the less impactful it may be.
Consider other means of communication: If a considerable amount of time has passed and you feel the belated wish might be awkward, you could consider reaching out in other ways, like sending a handwritten note or giving a small gift, expressing your belated birthday wishes.
Ultimately, the most important thing is the thought behind your gesture. If you genuinely want to wish someone well on their birthday, a belated greeting can still be appreciated, even if it's a little late. Just be sincere in your message, and they are likely to understand and be grateful for the sentiment.