Technically, you'll start to see results right away; however, it takes time for a campaign to "normalize" to the point of understanding how it will perform long term.
Most agencies will count a call that lasts 45 seconds or longer, for example, as a conversion. But we try to minimize false leads by hiring a designated team to listens to the calls that come in from an ad and give it a score. Our call rater team also gives us the added bonus of helping us to uncover any issues that might be negatively affecting conversion rates during the calls (like an automated answering machine that is confusing people, for example, or low quality leads who are asking for something different than what your business sells, etc.).
If you have a large customer list, and if your Google account is eligible for "customer match," then you can implement some cross-selling and retargeting campaigns. Because the audience size is limited and much smaller than an audience found on Google, we won't need a large budget for this strategy. After about a month or two of gathering conversion data, we can also create a "similar audience" based on the customer list. Similar audiences are often effective, and we will incorporate it into our proactive targeting campaigns.
Wait a month or two (because you need to have good enough performance from one platform before testing new networks because it’s always an investment to start a new network and therefore a new learning period) and then launch a Facebook remarketing campaign.
A smart approach would be to work your way to the top by generating affordable leads in a short amount of time with a small amount of budget by bidding on lower position, higher quality keywords. Then start to scale.