After recharging the LiPo akku over the last night I started the test again. I increased the sleep time to 10 minutes to get better compare to a test before. This test was documented at https://drive.google.com/drive/folders/0BwUOc1bstFZ9Zm1JbUlORDNua2s. Under these conditions I got a runtime of 48 days from February to April.
You can follow the measuring results at http://www.ckuehnel.ch/AM2315@NODEMCU.html again.
I added a weather widget from kachelmannwetter.com to see the correlation between weather situation (means sun) and the voltage of the LiPo akku.