How long does it take to gain certification?

You can take as long or short a time as you want to gain your certification.  You can study the knowledge part of the certification online or via home study in a day or a week, depending on how much time you can devote to it.  Then you can get your confined water and open water certification within three to four days if you wish.  Or you can take longer.  Most dive instructors have flexible schedules that will allow you to schedule your training in whatever way works for you.