Average lifetime for 26inch spokes I've been riding my bike for about a year and a half now and the back spokes are starting to snap, 2 now. I want to know when was this bike bought from the original owner since the spokes are stock spokes and it is second hand, it would be easy to know. All I do is just wheelies and bunny hops so maybe it might wear off more faster. Also, should I replace the spokes asap?
From the info you've given it's hard to tell how old your bike/the spokes are.

If the spoke tension is too low/if there is too big a difference in the tension of the spokes in the wheel this can cause premature spoke failure due to uneven force distribution over the spokes in the wheel. Make sure to have proper spoke tension/tension distribution to prevent premature spoke failure.

To answer your other questions:

1) Yes the wheels will not last as long when doing bunny hops and other such 'stunts' (such as riding down stairs, jumping off things etc.)

2) You should replace your spokes as soon as possible, if you don't and keep riding there is a significantly higher chance of more spokes braking (since the force is distributed over less spokes)