Female bull terrier is on heat

Question
my female bull terrier is on heat. please advise how long does it last.

Answer
Anisa,
All dogs are individuals like we are, the usual is about 3 weeks.  They usually start with a bloody discharge which lightens some time during the second week.  It may then go back to a bloody discharge or remain light in color, but they are still in heat.  The thirteenth day is the fertile time in 90% of female dogs.    There are cases  of females being fertile as early as 4 days and as late as 25 days.  So keep her confined during the whole time to prevent any unwanted matings.
Nancy