Leave it on to start and set up an advanced custom report in Google Analytics to test your conversion rate by time of day. Here's how:
(Note, this mainly works for web forms, not phone calls since you can track them as Goals in google analytics)
1. Set up a goal for a web form conversion in google analytics.
2. Click "Custom reports" in the left hand side.
3. Create a new report called "Hour of day Conversion rate"
4. For the top part, drag the blue box called "goal conversion rate for Goal X (Goal X being the same # goal you set up for the form conversion) from the metrics section to the blue area on the right.
5. In the "dimensions" section on the left open "Visitors" and drag "Hour of the day" to the right for dimensions.
Save the report.
Let that sucker run for a few weeks (depending of traffic volume) and then you can have good data on your conversion rate by time of day. Adjust bids accordingly.
Man, i'm just giving you oodles of goodies for this plumber guy as of late....