I think I drop “for” when I’m in a hurry, or more likely, just lazy. But I also use it. It probably depends on blood sugar, brain chemistry, time of day, what I’m thinking about at a given moment, or maybe the varying effects of cosmic-ray bombardment on my brain.
As to the present perfect, “I’ve warmed up. . . ,” I think I might say that if someone advised me to warm up before doing exercises, e.g.:
bob. You know, you should warm up before you lift weights.
xp+10K. But I’ve warmed up for an hour. Besides, I have no intention of lifting those weights. They look very heavy. I’d rather just warm up and look as if I’m about to lift some weights.
I might also say, “But I’ve been warming up for an hour,” which is probably preferable in the view of most grammar folk, but again, I’m lazy, and you got all these cosmic rays, etc.
Really, I’d probably say, “But I just warmed up for an hour.” Then again, if I’d been warming up for an hour, I probably wouldn’t say anything. Too tired.