Question: Within our team we would like to do load tests in a CI process. For this we want to use either Jmeter or just a comparable tool.
Jmeter offers u.a. the possibility that one can plan and execute a test pipeline for a load test via plugin.

The question we ask ourselves, how often should we perform this load test but in the development accordingly?

How often do you use your load test in your planning? Weekly? Several times a week?

Project details:
Webpages
Java, Javascript, Angular

Answer: 
IMO, for every major code change, you need to execute load test. You can decide either you can execute a load test as part of sprint closing/feature closing.
Make the load testing part of your performance suite so it runs automatically when you push branch commits to your CI system and fails when user experience deteriorates. The level at which that happens, e.g. slow slow responses is something you will need to define.

I also recommend you consider creating functional tests and then making them part of your performance suite so that you are stimulating actual user usage patterns. For example a selenium suite that fills forms.

jMeter is the clear leader in the field but it is an old clunky tool. Consider rolling your own, for example with Ruby which gives you thread management and the Benchmark library