Use "how long" when you are asking for a specific period of time. Ex. "How long have you been in California?" "2 years." Use "how soon" the same way you would use "when". It is asking about a point IN time. Ex. "How soon will my package arrive?" "It should arrive next week."

How long is used for length and time. How soon is used for time. Examples: "How long will it take to get ready?" "How long until we can go shopping?" "How long will the trip be?" and "How soon can I expect a card?" "How soon until the pizza arrivies?" With "long", it can mean two different things, but with "soon", it sounds a little impatient. You can pick what sounds best to you, or what you hear people around you use.

"how long" is used for lengths [distance or time] so we talk about the difference from beginning to end: How long is a football match? 90 minutes. How long is a football pitch? About 100 metres. "how soon" is only interested in the end time. It can either be the length of time from now or just the end time itself: How soon will the match finish? In 20 minutes. At 5 o'clock. or: How long has the match got left? [changed the beginning to the present moment] 20 minutes.

You can use "how long" for both of the things that Wendy mentioned. You could say "How long until my package arrives?" But you can only use "how soon" for asking about a point in time, as she said, "How soon will my package arrive?"