The first goal of any search engine optimization strategy is to get your web pages indexed. But even before that can happen, you need to get the search engine crawlers to visit your website. Depending on the search engine or directory and the overall circumstances (how you invite and solicit crawlers), that first visit could take days, weeks, or even months.
And while it’s true that the initial crawler visits can be somewhat unpredictable (or take a long time in coming), once the ice is broken, future visits can be controlled to some degree. Basically, the more frequently you update your pages, the more frequently the crawlers will show up on your website doorstep.