How long should roast pork be cooked so that it stays light pink?

The important part of cooking meat to a safe range is a combination of time and temperature. The ranges that you often see from people like the USDA are a temperature at which everything nasty dies instantly. This is a much more definitive test than saying that you need to reach a temperature and hold it for a certain period of time. The US government has a great table showing time and temperature for cooking pork safely (just scroll down a little from the link). As you can see, it's safe to cook meat well below the often given 160F. In fact, it's instantly safe to eat at 144F. I assume that the reason for the 16 degree buffer on the often stated temperature is just to make sure people don't run afoul of the magic number.

What you must understand is that, at these temperatures, it's imperative that you don't mess about with the amount of time. They must be held for at least the time given. But, 140F only needs to be held there for a minute. Once you've got something all the way to 140F, residual cooking should hold it there for at least 60 seconds. Just don't play this one fast and loose.

Cooking something like a roast is slightly more complicated than just getting the meat to temperature though. Generally there's a lot of fat and intramuscular tissue that you want to rend and denature respectively. That's what makes the meat so juicy and tender. While this will begin to happen at lower temperatures, it takes longer. So the meat may not come out as tender or flavor full as you'd like.

I would suggest a little experimentation. As mentioned, the thermometer will be a huge help if you want to be able to do this consistently (due to varying sizes of meat). I'd throw it in the oven and run some tests. I usually do Boston Butt to 168F, and this comes out with some pink parts, but I'm not sure how pink you want the meat (or how much of it should be pink). I'd recommend starting at about 160F and then moving up or down in temperature based on how the meal comes out.

The latest USDA guidelines recommend internal temperature of cooked pork to be 145 degrees. This applies to whole cuts only, not to ground meat.

You can read a summary of the changes online.

The USDA recommends an internal temperature of 160ºF (about 71ºC) for pork to be considered safe to eat. The exact time and temperature for your oven will depend on many factors, so always use a probe thermometer to check. If you're worried about it getting overdone, start checking the temperature a while before you expect it to be done (based on your previous experiences). There are also thermometers which you can leave in the oven, and have a remote part that can sound an alarm when a certain temperature is reached. Also note that the internal temperature will continue to rise a bit after you remove the roast from the oven, so you may want to aim for a few degrees below your target temperature.

210ºC (410ºF) does seem too hot to cook a roast, so I'd aim for something on the lower end. Whatever time/temperature combo you use, it's always the internal temperature of the roast that matters, both for safety and for taste.

I'd start it at 425F(210c) and then drop it to around 325F(160-165C) as soon as I put the roast in: starting it hot often gives better results than keeping it the same temp the entire process.

After that, you're going to want to cook it 20-30 minutes per pound (if it's a butt roast, you might bump that up to ~40 minutes). I'd start checking it with the thermometer at the very bottom end of that range, and take it out of the oven before it hits your target temperature.

I do NOT recommend 160F(71C) degrees as your internal temp, unless you like shoe leather. The USDA believes everything should be cooked to a cinder: trichinosis and salmonella and most other common nasties are killed at around 140F(60C) (144F is the instant kill number, but it's pretty hard to get your meat to 140 without it staying there for the 60 seconds that would be needed to kill everything). You should take the pork out at an internal temp between 140F and 145F degrees, to insure that the internal temp hits 150F(65C), which is a good safety margin.

Internal temperatures will continue to rise after the roast is removed from the oven. If you remove it at 160F or 165F it'll be 170-5F before it stops cooking, and that's flat inedible IMHO, and in no way pinkish. If you feel that you must wait until it hits 165F, take it out of the oven at 155F, and it will get to 165F before it peaks.

(Insert disclaimer about undercooked food blah blah blah...To be 100% safe you should cook it to 1000 degrees, then snort the ashes)