i jsut started a Blueberry Gum from G13, says 6-7 weeks flower (50 days), so around probably 7-8 weeks, thats pretty short. jsut stay with indica plants for the shorter flowering time.
you not going to find a strain really other than autos that will go shorter than 7-8 weeks. you can always harvest early but then the quality will be so-so at best.
more info in the blueberry gum
Sex : Feminized
Type : Indica
Flowering : Photoperiod
Genetics : Blueberry x Bubblegum
Flowering Time : Short, Medium
Outdoor Harvest : 50 Days
Height : Medium
THC Level : 15% - 17%
Characteristics : Sweet Bubbly Juicy
EDIT:
also got a Critical Mass 33 from CH9 going which is pretty short growing time...
Can be grown outdoor & indoor
Flowering time: 7-8 weeks
Indoor yield: Up to 120 gr indoor with 25-35 days of veg
Outdoor yield: Average of 400 gr outdoor or more depend on vegetative time and season, up to 600 gr
Lineage: Critical Mass x Jack33
Bouquet: Strong, peppery, rich and intense aroma
Type: Indica/Sativa