Do I have to buy Serum immediately after the demo ends?

Or can I try the demo now and buy Serum later?

You can try the demo now and buy it later :)
You are not forced to buy it as soon as the demo runs out.
(also the demo just stops producing sound after the time has run out and you can just restart the demo to try a bit longer)

Above reply is correct - the demo will run forever, just 20 minutes per use.

Above reply is correct - the demo will run forever, just 20 minutes per use.

Thank you.