The number 30 in stats is for positive outcomes - so in this case 30 clicks, not 30 impressions.
Be aware that this is a very general rule and can lead to a lot of bad decisions.
Take a lesson from political pollsters and put error bars on your numbers. Use 80% or 90% confidence intervals (meaning 8/9 out of 10 measurements you will be within the error bar range) if your range is too wide - say 5% CTR +or- 5%, then you need to run more impressions until the error bar gets into an acceptable comfort zone. Only you can define that zone.
As a general rule your margin of error will halve each time you double your denominator. So if your error bars after 1000 impressions is +/- 5% then your error bar after 2000 impressions will be +/- 2.5%.