If they do not mate, they can stay in heat for several days but most of the time they appear to stay in about 4 to 6 days, then come in heat again in about 2.5 to 3 weeks. This does vary widely and some cats appear to almost constantly stay in heat. From everything we have read, they can come into heat several times a year, but there is no mention of the average length of one cycle. This stimulates ovulation which ends the heat cycle. If they are not bred, they may go out of heat in as short a time as few days or stay in heat for a variable length of time.
Some cats appear to almost stay in heat continuously.
The way that cats behave during heat cycles is probably one of the major reasons that people have them spayed. Inc. Opinions expressed are those of Dr. Richards.