Newborns need to be checked and changed much more frequently than older babies, simply because they poo more often. For the first few weeks, you can expect some poo in almost every nappy / diaper, and should check every half hour or so if they're awake, probably changing once every 2 or 3 hours. If they're asleep I wouldn't bother waking them up to check / change.
Once they get a little bit older a cloth nappy / diaper will be fine for about 4 or 5 hours. If you aren't quick enough to change it then it starts to leak and you get damp patches on the baby's clothes!
NB as a happy cloth nappy user for both my children, I would NOT recommend using cloth nappies from the very start because the newborn poo ("meconium") is very difficult to clean and can stain the nappies easily. If disposables are an option for you they are vastly preferable for the first couple of weeks until the poo is normal.
Edit to answer your additional question about night times: In the night time you can add a booster pad to the nappy which makes it more absorbent (but also more bulky, which is why you don't normally use them during the day as well). Or if you're using prefolds, stick two prefolds in the same outer for the same effect.
In the very newborn days you'll be up frequently in the night anyway and just give the nappy a quick check then. After that you'll soon learn how long the nappy lasts before leaking and how often your baby poos. Definitely no need to set an alarm and check. There are really only two ways things can "go wrong" (if you haven't checked often enough) - either the nappy leaks, in which case just clean up and check a bit more often next time, or the baby ends up with a sore bottom from being in a dirty (as opposed to just wet) nappy, in which case barrier cream can help.
One particular recommendation is to make up the bed / cot as follows: mattress, waterproof layer, sheet, waterproof layer, sheet. Then if there is any leakage in the night you can strip off the top sheet and waterproof, leave them to be washed in the morning and go back to sleep on the second layer without having to re-make the bed!
Two general points: first, trust your instincts - you seem to be very unsure about quite specific aspects of all this and really, I promise, it will all fall into place once the baby is here! And second, you will inevitably get leaks / containment failures / sore bottoms etc at some point - just clean up and move on, the world won't end :-)