What should my boyfriend do if he often doesn't return messages for a long time?

It doesn't help to be anxious. If he doesn't reply to your message, you should know the reason first.

If he doesn't take the initiative, doesn't know how to find a topic and doesn't reply to your message, then you need to take the initiative to start a topic when sending a message, instead of waiting for him to reply to your message.

If he doesn't send you a message because he is busy, it also needs you two to communicate how to balance your career and love.

If he doesn't want to send you a message, then you need to have a good talk, and good communication can maintain the continuation of a love.

I hope I can help you.