Most bitches will have a couple of seasons a year and it can last up to 14 days - although this is changeable with every bitch - and she won't be receptive for all that time.
Personally I would suggest forgetting about breeding from her if the sole reason is based on an old wives tale. There is nothing to prove that any bitch suffers from not having a litter and it's something that has to be researched and taken seriously if it's to be considered.
IMO there are too many unwanted dogs as it is. Have her speyed and enjoy life with her without the worry of an unwanted pregnancy that will cost you time, money and possibly your bitch. She won't be any worse of for it.