hyphenation - Correct use of endash in range of minutes

I am currently working as a web developer, and will occasionally be asked to update a website. A "client" just send me an update containing this text:

A 15-30-minute waiting-period is required after each injection is given.

Notice the use of 2 hyphens, which I am sure is not the correct style to use. I have been taught to use en dashes to separate ranges of values, such as 15–30, and also to add word spaces if I feel the en dash runs into the words on either side. However, I also vaguely think I was taught to put a hyphen between a phrase like this: "… going on a 30-minute walk."

If I follow both of these "rules", I will end up with a sentence that looks like this:

A 15–30-minute waiting-period is required after each injection is given.

As you can see, this only differs from the original because of the en dash between 15 and 30. However, I still think that looks weird. I think that it ideally would look like this:

A 15–30 minute waiting period is required after each injection is given.

Note: I also removed the hyphen between waiting and period, as I don't think that should be there either.

Questions

Question 1:
What is the proper way of rendering the first part of the sentence? Is it an en dash between the numbers and then no hyphen between the last number and the word minute?

Question 2:
Should the words waiting and period have a hyphen between them?

The reason that this question doesn't answer my question specifically is that while I know the differences between hyphens and dashes, the way the sentence is composed lends itself to confusion. Having an en dash in the word before a hyphen seems rather strange.

Thanks!

Answer

You are right on both counts and I like your version of the sentence the best. There is no need for "30-minute" but it is acceptable and "15- to 30-minutes" is a fine suggestion by @FumbleFingers. I used thepunctuationguide.com as my reference.