To start you'll want to focus on Broad Match Modified keywords. They are the most expensive (only temporarily) but will allow you to cast a wider net while you figure out which keywords directly lead to the most sales. The length of this phase depends on the search volume your keywords are able to generate so its hard to pin point an exact time frame but generally 2 to 3 weeks should be enough time to gather a reasonable amount of data to get started.
Focus on converting the keywords you know are generating clicks into Phrase and Exact match which you can find by running a series of search query reports. People change how they search for your products online so the "data gathering" phase never really ends but as you transition away from broad match and narrow your focus altogether your CPCs will lower and you can start to better optimize.
If you would like to talk more you can reach me at email@example.com and I'd be glad to help.