This is my little one's favorite kind of play and we do this nearly every day. He likes to have his face sat on so that he can't breathe. Because I am always in contact with his face I can feel any change of expression at all, and we've developed a cue where he blinks to tell me to get up. It works extremely well, but we did not get there overnight; the first time probably lasted for a few seconds because I was terrified of crushing him. Now he'll go sometimes forty-five seconds, take just two or three breaths, and then go for more (but not for as long). He likes to be suffocated many times in a row with just a few breaths in between; I pay very close attention to the color of his face and the depth of his gasps; but we have been doing this for years; I do not recommend starting off like that.

I've never used something like a timer, but we did start very short and then slowly build up to longer amounts of time, until I became confident enough that I could read all of his body signals; chest motion, stomach motion, arms, hands, legs. He has passed out on me twice: both times because he didn't signal me to get up because he wanted to pass out (he was punished harshly afterward for scaring the crap out of me and for going too far without telling me he was thinking that! Ooooh, we had a long argument oh I was so mad).

I would recommend never using an object; you have the most control over your own body, you can react instantly if something goes wrong, and you can feel your sub directly. Like WyldWyl said, watch them like a hawk.

Okay, now that I've laid the safety on so thick you're probably scared out of your brain, keep in mind that this kind of play can be very, very rewarding. The level of trust that you build between you and your sub is beautifully deep; the level of non-verbal communication necessary will teach you new things about your sub that you never would have otherwise known.

So in short: Try it; start slow and small, pay attention, and reap delicious rewards.