I would say most professional pilots should be able to don a quick don mask in 9-15 seconds (provided its not submerged under magazines, sandwiches etc). The key is to permanently be primed to put it on at the first sign of trouble.
I would sometimes take it out and put it on my lap at the higher levels, so that I could be confident of donning it within a couple of seconds. It really isnt practical to wear one continuously on a long haul sector, but range would be very restricted on a global or gulfstream if it couldn't go higher than 410.
And that's before the question of whether you will actually be able to breath if the mask has gone into pressure breathing mode. If you have never practiced it then the chances are very low.
It feels odd at first, but again; not at all difficult, even on the first try.