It is not right to leave a 20 minutes limitation there but not having any option to remove it.
Simple question, if it is claimed as a feature in paid version. where is it?
There is no 20 min limitation in the lite version of this app, the 20 min limitation is only applied when you connect your device to the charger. If you are not connected to charger, you should be able to run it till your battery runs out. Paid version will not have this limitations.