My question is, How long do BCGs last?  I have a Colt Sporter A2 all original and i want to use this BCG in my new build until i get a BCG specifically for it. Is this recomended or not?  The Colt wasnt shot much so it doesnt have too much wear and tear on it and i was thinking i could just use the BCG until i get enough money to order a separate one for my new build.

Any input is appreciated.  Thanks!

The carrier should last indefinitely, and can be swapped with no ill effects. The bolt can be swapped, but is not recommended to do so.
The bolt and barrel extension wear together (mate) until this wear is nil. Once you swap a bolt to a new barrel (extension) this wear (mating) starts over.
Then when swapped again, the wear starts again.  

Swapping BCGs is done all the time, but is not recommended.
If you can acquire a bolt for each barrel, you are better off in the long run. HTH

Ricochet is 100% correct.

I also like to match bolt/BCG with the barrel maker...just part of my OCD-ness.  Point being that you frequently see posts with I have a V upper with a W barrel, X gas block, Y carrier and Z bolt and it won't run right?  What do I do?  More to the point, who do you go to?

In case you're interested, your bolt should also last around 5,000 rounds; more if you're willing to run it to failure (i.e. on a range gun).

I swap BCGs in my rifles all the time. Go ahead and use the Colt BCG for a couple a hundred rounds or so without fear. Shoot your guns and upgrade the parts if they break.

The total headspace range is .010”.
A lot of weapons start out in the middle or even beyond.
The mil-spec in my day allowed max .0028” headspace change in the first fifty rounds.
While break in wear may normally be less than the maximum the math seems obvious.
FYI a lot of people here do not check headspace and consequently would not have a clue they were getting into a problem until they started popping primers or worse.
Read the warning in the TM.