lol @ atleast 20 mins, i guess ur friends a certified mazda mechanic, eh? whats he doing in pakistan, he should be inventing some 2000 hp rotory engines in the states!
on a more serious note, your friend needs to get slapped for saying that, no disrespect, but thats what you get for spreading false info and misleadsing others.
NO- you do NOT need to drive atleast 20 minutes to avoid destruction, its a car and treat it like one, im sure the rx8 wouldnt sell at all, or mazda would produce it in the first place if that was the case. in pakistan we see many idiots owning rx8s who treat them like regular everyday corollas, you need to know that these are special rotory engines that require special care. you cant expect a ferrari to run on mitti kah thel or on a cultus' oil filter, same scene here. Basically, check your plugs, change your plugs after every 10,000 km, change your oil and put proper non synthetic oil every 2500 km, so as the oil filter and dont be a jack while driving it.
but yes, the engine will go after 100,000 km 
