If the petition is denied, then your husband would not be coming to the US.
There are many reasons why a petition would be denied or an alien is not admissible into the US. What you should do is review your specific facts with an attorney to check if the attorney can spot any potential problems for your petition for your husband.
For example, given that you are 18, you may not have enough income to meet the requirements of the Form I-864. Without enough income and without a joint financial sponsor, you would not be getting approval for your petition.
How long an approved petition would take depends on facts such as from which country your husband is coming. The US consulates at some countries process many visa petitions and therefore take longer to process any particular one.
Also, unless you are a US citizen, you husband would need to wait several years for a visa number to become available.
In general, a petition for a spouse of a US citizen may take about a year if no problem occurs. Sometimes, things are faster or slower.
Before applying for your husband, you want to make sure that he is eligible to receive his green card. You should review his immigration history with an immigration attorney. Similarly, you will have to prove that you earn enough money so that he will not become a public charge (you do this by filling out the affidavit of support). If you are not currently employed or do not earn enough money, you will need to have a co-sponsor complete an affidavit of support for your husband as well.
If you are a US citizen, and your husband is eligible, he will be able to immediately apply for a visa (green card) once your marriage petition is approved at the US consulate/embassy in his country. The wait time will vary depending on where your husband lives. If you are a green card holder, then the wait time will be much longer, as you will have to wait for there to be green cards available before he will be able to go to the consulate/embassy.
Please note that content of this reply does not constitute legal advice or create an Attorney/Client relationship in the absence of an agreement for legal services/retainer with my office. This is just a place to start in deciding what to ask your lawyer.