What is the longest possible time to wait for your girlfriend/boyfriend to get back to you?

Weird thing: my boyfriend said he'll get back to me when he completes his hospital night shift. He didn't contact me back since then which is very weird because he tells me daily when he reached back home. There has never been a day except once where he was out of reach for 17hrs. Otherwise its never more than a 6-8 hrs of break. He always used to make sure to keep me in his daily loop.

Now.. Initially the messages weren't delivering and the calls weren't connecting. Finally he's reading WhatsApp messages but not replying and the calls are connecting buy not answered either. He was very sweet when he sent the last message saying prepare well for exam dear... And that he'll get back to me. He has suddenly vanished. It has been a 30 hours now. What could it be? I feel sad and bad about it.

What is the longest possible time to wait for your girlfriend/boyfriend to get back to you?
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