Depends heavily on the situation. I'm assuming that you're referring to a five-ten feet range. A 275 FPS gun wouldn't hurt too much if you're using a .12 gram BB (What most low grade spring guns chamber), it would probably sting for a bit but would wear off in due time.
In the same instance, a 350 FPS airsoft gun chambering a .20 gram BB would hurt quite a bit, most likely leaving a welt or cutting the skin.
It should be noted that pain is an opinion, and therefore we can't really inform you -exactly- how much something would hurt you.
Hope this helps!
I think the 275 will hurt like being hit hard with a switch. And I think the 350 will probably break the skin, so it will feel like a little ball being pressed through skin and flesh. In other words, it will hurt a lot.
Point blank, pretty much anything above 200 would really hurt on bare skin. 350 might break skin, but will definitely leave a huge welt