This could soon look like Backpool illuminations!
OK
The MAX232 has a voltage doubler, so the output will be +/-10v.
However, that is not a very stiff 10v and will easily be pulled down to a lower voltage by small loads.
This is not a problem as the voltage range of a valid RS232 signal is quite wide, something like 3 to 12v for one state and -3 to -12 v for the other state.
As I said above, the voltage could be all over the place.
All depends on what is on the other end of the cable and what it is doing at the time. I am not sure that we can make a generalisation.
A couple of stray points from other posts in this thread.
It is not necessary to worry about unused inputs on the MAX232, there are pull-up/down resistors built into th chip.
Also when designing your "Light Show", be aware the RS232 TX and RX lines idle at -10v (ish) which is equivalent to a STOP bit and binary signal level "1" in asynchronous RS232 comms.
All this reminds me of an episode which happened to me many years ago.
Why did the Silent 700 go up in smoke?
A Silent 700 was a nice portable (in 1980) RS232 terminal with keyboard and printed output on thermal paper, made by Texas Instruments if I remember correctly. It was silent compared with a teletype or a dot matrix printer with solenoids driving wires which hit a ribbon to make the print.
So we had a Silent 700, as a piece of general use equipment.
One day I connected it to an RS232 output from a Ferranti Argus 700 computer, instead of the usual teletype machine (which was acting up as usual).
After a few seconds, clouds of magic smoke came out of the back of the Silent 700. "Oh dear me" thought JimB
The problem turned out to be that the Silent 700 had some diodes or similar wired to the RS232 connector, presumably to provide some kind of overvolt or ESD protection. These "diodes" conducted at about 8volts.
The Argus 700 RS232 card however put out a rather "stiff" 12volts on some of the handshake lines. The diodes did not stand a chance!
The A700 carried on doing its thing and I just disconnected all the pesky diodes in the Silent 700 and it carried on doing what it did best, much to my relief.
JimB