I hope to hear from you soon.

"I hope to hear from you as soon as possible" - Polite but firm. You can use this with anyone. "Please get back to me right away" - Firm but a little bit less polite. "Call me as soon as you get this message" - Very firm and not polite if you don't know the person. It's a message a boss or the police, or your mother-in-law would leave for you. "Please respond ASAP." (ASAP means As Soon As Possible) - This is a blunt and informal way of asking but it is not rude if you're talking to someone you know.

It's not really right. The closest I could get to this would be, I think, "I hope to hear from you as soon as possible." Alternatives might include, "Please let me know as soon as you can."

Looking forward to hearing from you 👍🏻