[navik1]

In the reaction
CH2=CH2 + HBr -> CH3-CH2 + -Br -> CH3CH3
from the middle one to the the third one.
i asked my teacher, but the only other thing he knew about it is primary carbocation are more reactive than secondary or tertiary.

[Doc Oc]

That 2nd reaction won't happen, the bromide anion will attack the carbocation and you'll end up with the alkyl halide.

[navik1]

the reaction is
CH2=CH2 + HBr -> CH3-CH2 + -Br -> CH3CH2Br
and not the other one
and in the second reaction the CH3-CH2 is positively charged

[Doc Oc]

Well, to answer your original question, not long at all, especially in the presence of a good nucleophile like Br-.  There's some factors that can help to stabilize it such as using polar solvents, but in general they are very quick to react.