Difficult to estimate as there are always optimizations which can make real-time rendering orders of magnitude faster, for almost the same work.

But naively this is 30 min per frame render time, which is 1800 sec, wanna get that down to about 1/25th of a sec, that's a 45000x improvement in computation required. How long will that take? Well assuming GPU's get twice as fast every year, then it will take still 16-17 years! Luckily we have those shortcuts.

However it's possible we may have a quantum leap in computer performance if the new Memristor architecture ever takes off, which could speed things up.