The thing is the draw the UPS will take isn't just based on the wattage of the PSU, it's more on the power it's drawing at the time IIRC - ie 1000 watt PSU in one system could be drawing 1/2 the power another one is depending on the components and what they are doing.
I have a Cyberpower 1500VA on my primary system, and a couple cheaper ones on my other ones. The "good" 1500VA/900watts one, at half load claims 11 minutes run time, and at max load only 2. I tested it with my PC, just sitting idle really, installing a program to simulate a windows update in progress while a power failure happened, and it ran over 20 minutes before I said screw it, and plugged it back into the wall.
Semp is right, the monitor draws a pile of power, but speakers can draw a lot too, depending on the type etc, so don't plug those into your UPS battery ports IMO, as they aren't needed like the PC/Monitor are in an outage. Most UPS have their slots/ports split, 1/2 battery backup/surge and the other half just surge, so make sure you plug in only the PC and monitor into the battery/surge if you can, or remember to yank the uneeded plugs if you lose power from the battery plug ins on the UPS to keep from unnecessary power drain.
Overall for what you need in your OP, if you have an average system/monitor, and want say 15+ minutes, you'll have to get a reasonably decent one, a 1000VA/600 watt should run around 100$USD for a good quality one, and that'll do the trick IMO. I'm not Captain Electricity at all, so probably others will know more technical stuff, but just based on mine and my time test, I think this should be good enough for your purposes.
