You should spend as much as Google is willing to take and as much as you can spend while still seeing a positive ROI. You don’t want to just be throwing money after bad keywords or targeting the wrong areas, but as long as you have all of the components in place to be successful, you should be spending as much money as they’ll take, as long as people are still clicking on the ad and converting into new patients. As long as you have room for those new patients, you should be trying to fill up those spots in any way possible.
One thing I will say is you can’t make people wait too long from when they click on the ad to when you have an appointment available for them. If there’s no availability for a couple of days, a week, or multiple weeks, those people will find someone else. The reason why they were on Google in the first place is because they’re looking for a solution. If you can be that solution in a short timeframe, I think you should be spending whatever it takes to get them into your practice.