I think that will have to depend on several factors.
(01). Room Size.
(02). Room Acoustics.
(03). Speaker Type.
(04). Speaker Size.
(05). Speaker Efficiency.
(06). Speaker Impedence.
(07). Type of Material That You'll Be Playing.
(08). Listening Position.
And in some strange way, all of these parameters corelate with each other one way or another. But all things being equal and all, I would say that if you are driving small, but high quality speakers in a small room, then I think that 100 W/P/C should be more than sufficient. A larger room with bigger speakers using the same parameters as a smaller room is going to demand more power than smaller one will. Using reasonably efficient speakers in a larger room, I think that 200-to-250 W/P/C is quite a bit even in a larger room. I think you have to think about the top six parameters that I have just listed when it is time to select an amplifier. And just an endnote with regards to speaker efficiency, when you double the power of an amplifier from say....... 100 W/P/C up to 200 W/P/C, remember that doubling the power an amplifier delivers does not net a doubling of loudness. Doubling the power of an amplifier will only net you a 3dB increase in loudness. If you want to double the loudness capability of your speaker system (and that is provided that either you or your speaker system is up to the challange), you are going to need a tenfold increase in amplifier power. So in saying that then, you will suddenly need to go from 100 W/P/C up to about 1000 W/P/C before you can tell any appreciable difference as far as system volume is concerned. Some amplifiers will also have anywhere from 1.5 to all the way up to 6dB of dynamic headroom. And that extra amplifier power an amplifier has on reserve is not meant to increase system volume per se, but it there to reproduce stuff such as gunshots, car crashes, explosions, etc.... effectively without the amplifier otherwise running out of power in order to maintain sound quality at the same time. But I will say that in most home theaters, most home theaters tend to utilize subwoofers for this very reason (to reproduce the heavy stuff such as helicopter engines, car crashes, automobile engines (check out "The Fast and The Furious..... you'll know what I am talking about then), gun shots and explosions). Bass tones in a home theater require tremendous amounts of amplifier power (sometimes, thousands of watts..... but in many cases hundreds (and the upper hundreds at that)), that may tax a lot of receivers and amplifiers, causing a lot of them to overload, overheat, and eventually, shut down, which is the very reason why powered subwoofers are often used in home theater systems.
So with that said then, in smaller rooms with smaller and efficient speakers and a powered subwoofer, and have a nearfield position, 60-to-85 W/P/C ought to be enough. On the other hand, if you have a VERY large room with bigger and demanding inefficient speakers, listen to a lot of demanding bass heavy material and your listening position is farfield as opposed to nearfield, then you're going to need a ton of amplifier power (which means the amplifier is going to VERY large, VERY heavy, and yes...... it's going to cost about as much as an automobile, which means it's going to be VERY expensive), and in that case, even 200 W/P/C is going only be scratching the surface, which to put in layman's terms, that won't EVEN be enough.
My H/K AVR-210 A/V Receiver is rated to put out 45 W x 5 in surround sound. But being that it is a high current design, it will probably equal to about 100 W/P/C coming from something like an entry-level Pioneer or Sony. I am considering upgrading to a Yamaha RX-V740 which is rated to deliver 90 W x 6 into 8 Ohms.
Something to think about the next time you shop for your next amplifier.