It's not a time based thing; it's on a promotion system. Full members can vote for or against the promotion of a user based on their activity on the Nexus. By taking the time to submit an introduction, experience reports, or asking quality questions that spark conversations, the full members will generally vote in favor of your promotion. If you ask a lot of questions that could be easily found by searching the site, people often do not vote or vote against a new user. The votes can be changed, so if you have downvotes, but change your attitude, members can change their votes to vote in your favor.

Good luck, and welcome!

P.S. Filling out the OPTIONAL questionnaire can help or hurt this process. If you give thoughtful, researched answers, people will notice that and vote in your favor. If you just give a lot of "IDK" or "I don't care" type answers, people are less likely to vote. Remember it is optional, so at any point you can stop taking the questionnaire and pick up where you left off on a later date.