It depends on a lot of factors, no name just a few (in order of the "power chain":
The output voltage of the PSU under load
The quality of the power cable
The condition of the main polyfuse F3
How much current the R-PI (Broadcom SoC) itself requires at that moment, depending on what it is doing.
How much current the LAN9512 is consuming, (depending on wether ethernet is turned on, and how much communication bothe Ethernet and USB is going on) and whether the voltage its getting is enough for normal operation.
The condition of the USB polyfuse (either F1 or F2 depending on the port used)
The quality of the USB to PS/2 adapter
The current consumption of the USB to PS/2 adapter
The current consumption of the PS/2 keyboard (may depend on for example capslock and other LED's)
The minimum voltage the PS/2 keyboard will keep working with, which normally would be 4.75V, but if you are lucky it could be less.
Any problem in this chain could lead to failure somewhere.
For example lets assume the PSU is of excellent quality and is actually outputting exactly 4.95 V, and the cable is of reasonable quality and drops only 0.05 Volt, the polyfuse is also normal, and is just 0.2 Ohm. The PI is running normally and is busy with a mondane task like running a browser, it means the LAN chips is on, and Ethernet is enabled, the LAN 9512 chip is drawing its normal 230 or so mA from 3V3 (which means its drawing that same 230mA from 5V also) The rest of the board draws 270 mA or so from the board for a total of 500mA. No problem for the polyfuse, its drop is normal at (0.2 x 0.5 = 0.1 Volt), if no other current is drawn we have a perfectly healthy 4.95 - 0.05 - 0.1 = 4.8 Volt, perfectly OK for the system (for example the LAN9512 chip runs on 3V3, from a drop down regulator (NCP1117-3V3) that works fine with input voltages just 1.2 Volt above their output voltage, so 3.3+1.2 = 4.5 V is enough.
So far so good, lets see what happens next.
The raspberry PI's USB polyfuses are rated 140mA, unfortunately that means they can have a resistance between 3 and 6 Ohm, when they are in fine condition, once tripped, (and recovered) they can be higher, people have reported values up to 8 Ohm. Lets say they are average and 5 Ohm, lets also say that the keyboard is drawing say 50 mA, and the USB adapter an additional 20mA, that means they draw a current of 70mA through the USB polyfuse, first that means that now the PI is drawing not 500 but 580 mA, and so the calculation will be slightly lower, because the PSU is loaded heavier, and the USB cable and input polyfuse drops increase, but lets ignore that, and the fact that a mouse is actually also drawing another 50mA or so.
What happens with the output voltage, well the drop over the 5 Ohm polyfuse will be 0.07 x 5 = 0.35 Volt, so the USB to PS/2 adapter and hence the PS/2 keyboard will receive 4.8 Volt minus the polyfuse drop is 4.8 - 0.35 Volt = 4.45 Volt.
Note that if all goes well, assuming no bad PSU, no bad powercable, no unusual input polyfuse resistance, no unusual current draw (I forgot, the HDMI cable could also draw 50mA, maybe much more when you are using an active HDMI to VGA converter) and an average USB polyfuse leaves us with just 4.45 Volt, just 0.05 volt over the absolute minimum a regular real to spec USB keyboard is happy with. But a PS/2 keyboard was designed long before USB appeared on the market, and was designed according to normal rules for 5V logic, which means only a minimum of 4.75 was required, with just 4.45 Volt all bets are of, and that is if all other potential problems do not apply!
phew, that was a long one.... I hope this clears up what the situation with the R-PI is.