I'm 26 weeks pregnant with very thin uterus i'm on progesterone injection since i was 20 wks, how long can my uterus hold this pregnancy . thanks?

Impossible to tell: That's an impossible question to answer. Unless a woman has had previous uterine surgery, or has a congenital abnormality of the uterus, there's no such thing as a 'thin' uterus. The cervix can be shortened, maybe that's what you are referring to? If so, then some women with shortened cervix go all the way to term. If threatened preterm labor, make sure you've had a course of steroids for baby.