Cats are induced ovulators which basically means they stay in heat until mated (or spayed). They may not be calling the entire time but tend to cycle in and out of the calling stage. It's really not pleasant or healthy for them to be in heat all the time, so cats who aren't part of a responsible breeding program should be spayed.
Also, them being induced ovulators, they CAN get pregnant without heat too, if some fertile tom mates them. Although it is not that common - toms are usually not very interested in them this way, if they females arent in heat. But it happens, especielly dominating toms may do it.
I'd say an average of 5 days, and it can come back every 2 weeks. For my cats personally (I'm a breeder) they get in heat for 5 days, then have a 7 till 14 day break, then start again. Some heats are worse than others. They are currently in their winter-break, so no heat atm But it should start again soon.