You can, if you are lucky, see something as early as 5 weeks.
6-8 weeks but like Clysta said before 8 weeks you may just be in for unneeded stress if you don't see anything
6-8 weeks is usually the best time to go. I would recommend not having too early of an ultrasound though. Dates can be off and if done too early you won't see anything and it just causes unneeded stress and worry.