If I purchase land in an Opportunity Zone, do I need to build within a specific time to qualify for the tax incentive?

Functionally, you need to begin an earnest process to build the property within a year or so after purchase. You can't land bank for 5-6 years and then develop after such a long holding period. The rules effectively consider this abusive and wouldn't allow it. What you should do is come up with a business plan for the first 4-5 years after purchase and then execute on it; if you haven't taken concrete, affirmative steps to begin renovating or developing, you'll have a very hard time with compliance and risk disqualifying the QOF that beneficially owns the property.